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.
Spank The Monkey mini-game
In the Spank The Monkey mini-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.
Beefcake bonus
Eat every single can of Weight Gain 4000 in the Beefcake mini-game to see a bonus animation at the end.