Mario Circuit
Use a mushroom to skip the corner at the Chain Chomp, performing a stunt as you come off the ramp to gain another boost as you land on the other side of the bend.
Coconut Mall
There are several places where you pass a pair of moving escalators. One goes in the direction you are driving, and the other in the opposite direction. Pick the one which goes in your direction to receive a speed-boost up the slope. You will slow down if you drive over the other one.
Watch out for the cars near the end of the tracks; if you get hit bye one, you will lose your items and lose a lot of time. Avoid these at all costs!
DK's Snowboard Cross
After you land at the top of the mountain, you will encounter a ramp, and after that some very dense snow. You will slow down tremendously in this snow, so unless you have a mushroom, star, etc. avoid this.
Near the end, you will enter a half-pipe with some snowboarding Shy Guys. Avoid the dense snow by boosting at the top of the half-pipe.
Wario's Gold Mine
Stay on the track and watch out for mine carts once you enter the mines.