TIME SPLITTERS 2 best fps ever, and it even has a map maker for multi

also cant go wrong with:
mario sunshine
zelda wind waker
paper mario
super smash bros melee (good mindless button mashing)
sonic adventure 2 battle (voice acting is shit but w/e)
dark summit (snowboarding game)
tony hawk underground 1&2
cell damage (mindless violence)
prince of Persia games
metroid prime games
Luigis mansion (for something fun and short)
pikmin (if you have enough patience)
these are just some of the many good games on gamecube i can think of.
EDIT: just remembered, Ty the Tasmanian tiger, (only if you like Australian humor, tigers with boomerangs and dont mind bad voice acting, this is the game for you).