Try to lose racing levels several times and your opponents will also start to lose theirs. Once they do, try to win.
Pizza
In the Rugrats Food Fight levels go on top of the fridge to get the pizza.
Secret characters
Go into Blast, then press Down, Down, Right, Left, Right, Up, Left, Down, Right. Then, go to the character selection screen. Cosmo, Wanda, Patrick, and Carl Wheezer will now be unlocked.
Unlock the Bungi games
Highlight the Bungi game at the bottom of the game selection screen, then press Up(2), Down(2), Left, Right.
Unlock the Clam games
Go to Blast, then press Up, Up, Down, Down, Left, Right.