In toy box mode, you all realize that there are two diffrent racecar tracks?

Well go in the first one where it has the bowling and soccer. Then go near one of the race starter beacons and enter the race.

After that, once the countdown starts have the other player ram into the one who's in the countdown. Note this will not work if the countdown is over.

After that the player will be knocked out of their car. You'll be able to roam around and encounter some wierd glitches, but it is safe becuase I have done this numerous times.

Have fun!