Trials Fusion - each player chooses what they would like to race each other with: beginner bike, dirt bike, trials bike, peddle bike, pit bike, 4-wheeler, or a unicorn. Then race each other to the finish line.
I've made a couple local multiplayer tracks in it already that can be downloaded and played with your couch co-op buddies already. Otherwise there is a whole slew of ones that the game naturally supplies you with.