Mario, Peach and Yoshi Cars
There are there green pipes around the map. One at the abandoned airport, one at the bridges construction site near the railroad, and one at the edge of the cliff, behind the lighthouse. If you jump and park right above each pipe, and then move the camera to focus your car from the front, the pipe will warp you to a hidden garage where you can find special colored editions of in game cars, one for Mario, one for Peach, and one for Yoshi.