Eat every single can of Weight Gain 4000 in the Beefcake mini-game to see a bonus animation at the end.
Play as Cartman if you want a better variety of mini-games. He is the only character that can play the Rodeo and Beefcake mini-games.
In the Spank The Monkey game, notice that the monkey's butts become red when Mr. Mackey hits them, and become redder every time they are hit. If you forget the pattern, watch the order that the butts turn back to normal.