In the Donkey Kong's Jungle Parkway level, you can use a Mushroom or a Star to get up the cliff in the cave not far from the finish line. This can cut seconds off your time.
In the Wario's Stadium level, it is possible to use Mushrooms/Stars at certain points to be able to jump over the walls and onto further sections off the track. Using these can cut minutes off your total time, which can help as this is a long track.
I will submit others I find at a later date, when I find them.
Added by: Volke Dec 31st 2008, ID#2293