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. If you have good off road speed, you should do it even without a mushroom boost.
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 by one you will lose your items and a lot of time. Learning their patterns is essential for getting as many of the boost pads here as possible.
Don't forget about the shortcut near the beginning. at the point at which you are given a choice between two flat roads and an escalator, if you have a mushroom, take the rightmost path and go through the store with an open door, and you'll shave valuable seconds off your time. don't forget that the store floor counts as rough terrain, so make sure to use that boost.
near the end, when you are close to the parking lot, always take the high road. the jump there is significantly higher and gives much more boost if you do a trick.
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.