Mini Games
After you select the file you want to play, go to world 1-1. Then, hold select and press X, X, Y, B, A and release the select button. You now could play the mini games. If you win in the one player mini games, you get to have that item in the adventure (normal) game.
Secret Levels
Get 100% on all the levels in a specific world and the two question marks will appear and become fun levels!