Enter the following codes at the 'Cheats' screen:

citybeacons	Win everything
inflatedego	Big head mode
iwish		Unlimited time   
tearound	Always win
travolta	Power-up moves   

Extra boards:

Combat Board	Beat the boss on the UK course 
Speed Board	Beat the boss on the US course 
Trick board	Beat the Japan course  

Thanks to Revolution readers Gman10, Brian Milan, Homey G., K-Rock, and zombieman_3000!