In each level you will find a hidden vehicle. Try and look for them!
Mission set 1: Go to where mr. Burn's mansion is. Take the street down towards the golden mansion. A rocket car should be parked out front.Be careful though, it damages really fast.
Mission set 2: G up the flight of stairs to the monorail. you should be able to take the driver car. It isn't very fast though.
Mission set 3: Drive up to the 'C-Spanker' boat. Find a way in. Climb up one of the cargo boxes and you'll find a jet boat. It's fast and actually drivable on land!
Mission set 4: Go to the trailer park. Behind one of the trailers will be a quadbike.
Mission set 5: At the stadium, jump onto the fountain, then on the platform next to it. There should be a huge monster truck on the platform, the 'obliterator' or something.
Mission set 6: Go to 'Planet Hype.' Get on the roof and postion yourself above the car. Get off the roof and land on the car. You'll be able to drive it, a pink cadallac.
Mission set 7: Get on top of the Krusty Burger. There should be a very small R.C. car on top that you can actually get in! Also if you look close into it when in it, you can see yourself, just very small!