Low-poly time trials. Beat the medal times, then beat everyone else's. Log in to put your times on the board, or just start driving.
Each track is a single run from the start line to the finish, through every checkpoint in order. Miss one and the run does not count. Fall off and you restart from the last checkpoint you crossed - the clock keeps going.
Four medal times per track, from bronze up to author, which is a lap driven about as well as the track allows. Your best run on each track is saved as a ghost you can race against, and the fastest ghost on each track is there to chase too.
In multiplayer everyone drives the same track together. The host starts a countdown when people are ready; between races you can keep practising in the same room. Cars are solid - bumping is allowed and encouraged.