When you beat all the missions you will get a new car( it looks really funny) it's a car built by homers bro and to use it go do road rage or sunday drive ect. Get homer it say choose one family sedan or car built for homer
If you are playing multiplayer the best thing to do is to get homers brick car ( hold L1 + R1 and press: triangle, triangle, square, circle) go to evergreen terrace and go around to that bit by krabapels apparment and near Lovejoys house, there should be a 4WD track and a building up beside it with some grass. Go up the grass so the 4WD track is to the right of your screen, and go to the little dirt ledge and drive along there. Not many cars can get up there very easy.
Go into the springfeild mountains level and go to the pet cemetery and turn right and go though the other gates. Go to the top of the hill and you will find Mr burns Mansion! Do what ever you like trash his huge backyard or even just go in his house!