There used to be this Gameshark for PS1 that let you pause the game and watch over variables to see if anything changed. That way, you could figure out where in the code you can do things like get unlimited lives. Beyond that, I used to play with the NES Game Genie to try and make games weirder, like altering the gravity in Mario. Nowadays I don't bother, I have barely enough time to play as it is!