Wii Moo Moo Meadows - Oh those blasted cows. When you reach the area where the cows cross, use Mushrooms to bypass all the turns, including the long one at the end. And also, a Mushroom can be used to cut through the very last turn of the track!
GBA Mario Circuit - We've got 4 secrets here! Mario Circuit features many turns that can easily be bypassed by using a Mushroom to cut through! Cutting the first corner saves a lot of time! After you exit the Anti-gravity section, keep an eye on the left for a ramp behind some trees. Use a Mushroom to reach the ramp! The last turn on the track is covered with a line of tires. Use a Mushroom to bust through the tires! To the right of the finish line is small route with an Item Box and a speed boost.
DS Cheep Cheep Beach - This one's a serious beach...aha..ha...When you reach the arrow sign pointing to the left, continue into the water and use a Mushroom over the dark sand to cut through this entire section. You can also use a mushroom to cut through the last corner where you can see the mud!
N64's Toad's Turnpike - You can't beat a classic! Well not without a baseball bat or some other bludgeon....Keep an eye out for a truck with a Gliding ramp just before you go under the highway. Glide as high as you can to pass right over the entire highway.