NOTE: All credits for these codes go to Sleipnir17 and rileyk64, huge thanks to both of them. I just kinda stuck the box names together as necessary.

There's a chance I messed some of these up, so reach out to me if you run into issues.

List of available codes:
 
-Unlock Southern Island
-Get any item
-Make Groudon/Kyogre reappear
-Make Rayquaza reappear
-Make Regirock, Regice and Registeel reappear
-Make Hoenn starters reappear
-Make gift Beldum reappear
-Make Wynaut egg reappear
-Make the two fossils reappear
-Change trainer name to any legal name

Want more codes? Give a look here:
E-Sh4rk's code generator: https://e-sh4rk.github.io/CodeGenerator/index_rs.html
 
Actual codes:

-Unlock Southern Island:
Note: this code will unlock Southern Island by setting its flag and providing the ticket. Go go collect the ticket from the lottery lady in Lilycove Department Store, then check if the event is triggered by taking the Lilycove ferry to see if the flag is also correctly set.
You can safely use this code even if you somehow already have the ticket or you already have the flag set, you'll simply get the missing piece from the execution, thus unlocking the event (in case you already have the ticket you'll still get a new one from the lottery but that's fine, collecting a second ticket will not add a new one to your bag)
_______________________________
 For English versions:

 Box names:
  Box 1:  (f?UnoU…o) [one …]
  Box 2:  (.PnjR?n ) [ends with space]
  Box 3:  (EE♀GUm  ) [ends with two spaces]
  Box 4:  (EZ……o   ) [two …; ends with three spaces]
  Box 5:  (Q Qn'Ifm) [one space after Q; righty '; upper case i]
  Box 6:  ("!n Fzz ) [lefty "; one space after n; ends with space]
  Box 7:  (EE'IPm  ) [righty '; upper case i; ends with two spaces]
  Box 8:  (E'CPm   ) [righty '; ends with three spaces]
  Box 9:  (E       ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

 
-Get any item:
Note: execute this code filling the variable characters as shown below, then go collect the item from the lottery lady in Lilycove Department Store 
Instructions: choose the item you want and get its hexadecimal index: if you're aiming for a regular item you can check the index here: https://bulbapedia.bulbagarden.net/wiki/List_of_items_by_index_number_(Generation_III), if you're aiming for a glitch item then you already know its hex index. Said index should be a four digit value, ♥♦♣♠ (pad with zeros to the left if the index has less than four digits). Now choose the right code depending on your game language and fill the variable characters as shown in the code, according to ♥, ♦, ♣ and ♠
_______________________________
 For English versions:

 Box names:
  Box 1:  (zRUn?SRn)
  Box 2:  (EEE*……n ) [two …; ends with space]
* = F if ♠=0, G if ♠=1, H if ♠=2, I [upper case i] if ♠=3, J if ♠=4, K if ♠=5, L if ♠=6, M if ♠=7, N if ♠=8, O [upper case o] if ♠=9, P if ♠=A, Q if ♠=B, R if ♠=C, S if ♠=D, T if ♠=E, U if ♠=F
 
  Box 3:  (EE*D!n  ) [ends with two spaces]
* = J if ♣=0, K if ♣=1, L if ♣=2, M if ♣=3, N if ♣=4, O [upper case o] if ♣=5, P if ♣=6, Q if ♣=7, R if ♣=8, S if ♣=9, T if ♣=A, U if ♣=B, F if ♣=C, G if ♣=D, H if ♣=E, I [upper case i] if ♣=F
 
  Box 4:  (E*B!n   ) [ends with three spaces]
if ♣=C,D,E,F, then
* = J if ♦=0, K if ♦=1, L if ♦=2, M if ♦=3, N if ♦=4, O [upper case o] if ♦=5, P if ♦=6, Q if ♦=7, R if ♦=8, S if ♦=9, T if ♦=A, U if ♦=B, F if ♦=C, G if ♦=D, H if ♦=E, I [upper case i] if ♦=F
if ♣=0,1,2,3,4,5,6,7,8,9,A,B, then
* = I [upper case i] if ♦=0, J if ♦=1, K if ♦=2, L if ♦=3, M if ♦=4, N if ♦=5, O [upper case o] if ♦=6, P if ♦=7, Q if ♦=8, R if ♦=9, S if ♦=A, T if ♦=B, U if ♦=C, F if ♦=D, G if ♦=E, H if ♦=F
 
  Box 5:  (*/!n'"Rm) [righty '; lefty "]
if ♦=D,E,F or (♦=C and ♣=C,D,E,F), then
* = J if ♥=0, K if ♥=1, L if ♥=2, M if ♥=3, N if ♥=4, O [upper case o] if ♥=5, P if ♥=6, Q if ♥=7, R if ♥=8, S if ♥=9, T if ♥=A, U if ♥=B, F if ♥=C, G if ♥=D, H if ♥=E, I [upper case i] if ♥=F
if ♦=0,1,2,3,4,5,6,7,8,9,A,B, or (♦=C and ♣=0,1,2,3,4,5,6,7,8,9,A,B), then
* = I [upper case i] if ♥=0, J if ♥=1, K if ♥=2, L if ♥=3, M if ♥=4, N if ♥=5, O [upper case o] if ♥=6, P if ♥=7, Q if ♥=8, R if ♥=9, S if ♥=A, T if ♥=B, U if ♥=C, F if ♥=D, G if ♥=E, H if ♥=F
 
  Box 6:  (E       ) [ends with seven spaces]
  Box 7:  (E       ) [ends with seven spaces]
  Box 8:  (E       ) [ends with seven spaces]
  Box 9:  (E       ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]


_______________________________________________________________________________________________________________________________________
 
 
-Make Groudon/Kyogre reappear:
Note: this code will make Groudon/Kyogre reappear if you already caught or fought it. You should not use this code if Groudon/Kyogre is present since you'll not be able to check if the code worked properly
_______________________________
 For English versions:

 Box names:
  Box 1:  (gBUn/U…o) [one …]
  Box 2:  (CQns Rn ) [one space after s; ends with space]
  Box 3:  (EE…Ogm  ) [one …; upper case o; ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (…OQm F…o) [one …; upper case o; one space after m; one …]
  Box 6:  (…!ny…!n ) [one …; one …; ends with space]
  Box 7:  (EE'OQm  ) [righty '; upper case o; ends with two spaces]
  Box 8:  (E       ) [ends with seven spaces]
  Box 9:  (E       ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

 
 
-Make Rayquaza reappear:
Note: this code will make Rayquaza reappear if you already caught or fought it. You should not use this code if Rayquaza is present since you'll not be able to check if the code worked properly
_______________________________
 For English versions:

 Box names:
  Box 1:  (g?Unz……o) [two …]
  Box 2:  (.PnS Qn ) [one space after S; ends with space]
  Box 3:  (EE/Nfm  ) [ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (/NPm    ) [ends with four spaces]
  Box 6:  (E       ) [ends with seven spaces]
  Box 7:  (E       ) [ends with seven spaces]
  Box 8:  (E       ) [ends with seven spaces]
  Box 9: (E        ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

 
-Make Regirock, Regice and Registeel reappear:
Note: this code will make Regirock, Regice and Registeel reappear if you already caught or fought. You should not use this code if all the three Pokémon are present since you'll not be able to check if the code worked properly but it is fine to use it if some of them are present
_______________________________
 For English versions:
 
 Box names:
  Box 1:  (g?UnlE…o) [lower case L; one …]
  Box 2:  (.Pn  !n ) [two spaces after n; ends with space]
  Box 3:  (EEDOfm  ) [upper case o; ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (DOPm    ) [upper case o; ends with four spaces]
  Box 6:  (E       ) [ends with seven spaces]
  Box 7:  (E       ) [ends with seven spaces]
  Box 8:  (E       ) [ends with seven spaces]
  Box 9: (E        ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]


_______________________________________________________________________________________________________________________________________
 
 
-Make Hoenn starters reappear:
Note: this code will make Birch's bag reappear in its usual spot in Route 101. By interacting with it you'll be able to pick up one of the three starters. After that you'll enter the battle against Poochyena and after beating it you'll warp into Birch's laboratory. Your starter will be in your team or, if you don't have empty party slots, in your boxes
_______________________________
 For English versions:

 
 Box names:
  Box 1:  (g?Unm/…o) [one …]
  Box 2:  (.Pn  !n ) [two spaces after n; ends with space]
  Box 3:  (EE…Nfm  ) [one …; ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (…NPm    ) [one …; ends with four spaces]
  Box 6:  (E       ) [ends with seven spaces]
  Box 7:  (E       ) [ends with seven spaces]
  Box 8:  (E       ) [ends with seven spaces]
  Box 9:  (E       ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

_______________________________

_______________________________________________________________________________________________________________________________________
 
 
-Make gift Beldum reappear:
Note: after executing this code the Pokéball containing Beldum in Steven's house will reappear. Executing this code while the ball is already available will have no effect and should not be done since there would be no way to check if the code worked properly
_______________________________
 For English versions:
 
 Box names:
  Box 1:  (g?UnME…o) [one …]
  Box 2:  (.Pnp !n ) [one space after p; ends with space]
  Box 3:  (EEDJfm  ) [ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (DJPm    ) [ends with four spaces]
  Box 6:  (EEEEEEEE)
  Box 7:  (EEEEEEEE)
  Box 8:  (EEEEEEEE)
  Box 9: (E        ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]


 
-Make Wynaut egg reappear:
Note: after executing this code the old lady in Lavaridge City will give you the egg again. Executing this code while the egg is already available will have no effect and should not be done since there would be no way to check if the code worked properly
_______________________________
 For English versions:
 
 Box names:
  Box 1:  (g?UnME…o) [one …]
  Box 2:  (.Pnp !n ) [one space after p; ends with space]
  Box 3:  (EE/Jfm  ) [ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  (/JPm    ) [ends with four spaces]
  Box 6:  (EEEEEEEE)
  Box 7:  (EEEEEEEE)
  Box 8:  (EEEEEEEE)
  Box 9: (E        ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

_______________________________________________________________________________________________________________________________________
 
 
-Make the two fossils reappear:
Note: this code will make both the fossils reappear if you already picked up. You should not use this code if the fossils are present since you'll not be able to check if the code worked properly
_______________________________
 For English versions:
 
 Box names:
  Box 1:  (g?Unz……o) [two …]
  Box 2:  (.Pna Qn ) [one space after a; ends with space]
  Box 3:  (EE"Pfm  ) [righty "; ends with two spaces]
  Box 4:  (E FRm   ) [one space after E; ends with three spaces]
  Box 5:  ("PPm    ) [righty "; ends with four spaces]
  Box 6:  (EEEEEEEE)
  Box 7:  (EEEEEEEE)
  Box 8:  (EEEEEEEE)
  Box 9: (E        ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

_______________________________
_______________________________________________________________________________________________________________________________________
 
-Change trainer name to any legal name:
Notes: this code will change your trainer name to the name you write in box 5. This means you can only use names you can write with legal characters
_______________________________
 For English versions:

 Box names:
  Box 1:  (hCUn♀…!n) [one …]
  Box 2:  (BQnFHEn ) [ends with space]
  Box 3:  (EEF BuB ) [one space after F; ends with space]
  Box 4:  (mEEEEEEE)
  Box 5:  (*******)  [leave the last slot empty]                                                               *******=desired name
  Box 6:  (EEEF !u ) [one space after F; ends with space]
  Box 7:  (EEEEEEEE)
  Box 8:  (EEEEEEEE)
  Box 9:  (E       ) [ends with seven spaces] 
  Box 10: (     …oa) [starts with five spaces; one …] 
  Box 11: (…o      ) [one …; ends with six spaces]

_______________________________