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.
Eat every single can of Weight Gain 4000 in the Beefcake mini-game to see a bonus animation at the end.
Unlock all mini-games
To get all of the mini-games pick Cartman then when you go to the screen where you pick the number of rounds enter Up, Up, Left, Down, Up, Up. A list of mini-games should pop up, hit X to select one.