Cloudtop Cruise - Bowser's airship, flying though the clouds and riding up beanstalks...sounds like an LSD trip. Or you know...Mario Kart. You've got one trick here, just before you reach the finish line, notice two giant leaves on your right. Ride right onto them and perform tricks to cut through the last turn.
Bone Dry Dunes - This place has a lot going on. Use a mushroom to cut through the sand on the right at the very beginning. When the track splits in two at the start of the race, use a Mushroom to cut through the centre and to reach the ramp. If you take the path with the Bone Piranha Plants, use a Mushroom to cut through the sand leading to the ramp.
To reach the wooden path after the Gliding section, either use the path made of bones and start the Gliding section by jumping off at the top, or use the normal path and use the sand fountain to boost you up during the second lap. As soon as you exit the area with the Dry Bones, use a Mushroom to cut through the dirt on the right!
Bowser's Castle - Made entirely out of antigravity sections, this place has one trick. At the end of the track are several tight turns. Use a Mushroom to get past any of the three rough areas.
Rainbow Road - This Rainbow Road is home to the Mario Kart radio station! In the space station with the rotating floors, use a Mushroom on the inside of the area to cut through most of this section. Use the Gliding ramp on the right to land on the blue section of the split track. Ride straight into the boost bumper, but continue driving right off the edge. You'll land right onto another section of the track, saving you some time. You can also use a Mushroom to cut through the rough part below.