How to unlock secret levels
You have to find a shining box in a racing map thats places outside the normal race zone, there are three in the tropical and four in the jungle. Boxes wont appear twice on a same map (not race).
You should be steering out of a corner almost as soon as you get into it if you want to clock the best times.
Don't be afraid to nudge your opponents off the track. Some even have cliffs you can push them over.
With a car with fully upgraded off-road handling, you can take a much more experimental line through the corners.