One of my favourite games for the GBA is WarioWare: Twisted!, the crazy sequel to the original WarioWare, where you play through tiny little stages called microgames using the game’s unique motion-sensing cartridge.
You play through 15 different stages in Story mode to unlock different microgames. Every time you beat a set of microgames in Story mode, you get a souvenir, which can vary from an interactive toy to a song from the game’s soundtrack to a full-fledged minigame. You can also replay unlocked microgames in the Spindex mode.