In the desert wasteland, right by the bridges is a head that provides you with various car challenges. This includes:

* jump challenge
* hang time challenge
* distance challenge
* roll challenge

In your menu screens you can see the bronze / silver / gold levels for each one, to determine your aims. You should of course use the bridges to attain these, and a lot of them just take practice in determining the angle to launch yourself off the bridge.

It's a ton of fun!

Jak and Daxter 3 Walkthrough