Jump on Anything Code: 017136D7 (017135D7 in Yellow)Just DON'T GO OUT OF BOUNDS (instant crash) and DON'T WALK IN FRONT OF ANY TRAINERS. Walking ON Trainers is fine, mind. :V
Once I did something to make the map load incorrectly and turned all the sprites into Swimmers and Pokemon, but that's about all. Take a nice, scenic tour around Kanto! Pick up Eevee early! Buy a Magikarp! Enjoy the blissful peace of bouncing like Tigger.
(Irritatingly, I tried to Gameshark in the Dome Fossil but the scientist refused to take it. Bah humbug. Had to walk all the way to Celadon to get Pokemon for a playtest.)
I just used it to test some silly stuff without actually having to start a Yellow file, since I'm saving that for after I've ended one of my current Nuzlockes. I confirmed that I had the right Professor Oak codes, for one. For two, I managed to see Kanto's sights without boundaries... useful if I ever want to do any creepypasta or Missingno.-based stuff. (It is way, way too easy to murder the game by going out of bounds. Seriously, I had to take savestates because I kept taking wrong turns.) For three - yay, I know what Gameshark stuff is like now! :V