In Free Ride City Trial mode, go off the green ramp by the big waterwheel with the flight warpstar. Fly as far and as high you can then wait until you hit a barrier. When you land, you will realize that you hit a second barrier and have landed in the water past the first barrier. If you turn around, you will see a ramp. Go up, and you will be back on the normal field and the ramp that you just went up on will be blocked by a fence. There is another ramp by the volcano, which will also be blocked by a fence after you use it.
In Free Ride City Trial mode, go to the satellite (the thing in the sky that you get to by going inside the volcano or on top of the volcanic mountain and going inside on the yellow rails). Get on the Flight Warp Star (the orange star like the normal warp star) and jump off towards the nearest edge of land. Go as far as you can until you hit the real wall and fall. You are now outside the original barrier.
Get a very good flying vehicle, like Dragoon. Go on a ramp and fly directly up. You will soon start bumping around. After that, you will start going down. Keep holding Up. If done correctly, you will have left the level boundary.
