Note: This procedure involves changing a game file; create a backup copy of the file before proceeding. Locate the track you would like to play, but is locked (for example, "\TrackMania Sunrise\GameData\Tracks\Campaigns\Nadeo\Sunrise\Race\NightLife\Suburbs.Challenge.Gbx"). Copy it to the directory "\TrackMania Sunrise\GameData\Tracks\Challenges\Downloaded". Start the game and go to solo/single challenges. The map "Downloaded" will now contain the copied track, which now can be played without the Gold medal requirement.
An easy way to do stunts in the SUV is go off the corner of a ramp or crash into a wall or obstacle to flip uncontrollably. Note: You can stop the uncontrollable spins by holding [Right] or [Left] against the spin or brake. In the Lamborghini, speed up and go off into a wall or hold a direction to spinout. In the coupe-like car, it is nearly impossible to do stunts due to its low acceleration and speed. If you can get enough speed, do the same thing and go off the corner of the ramp or ram into an obstacle.