DS Tick Tock Clock - I don't really have much time for this track...my god they're getting worse. If the clock hands are aligned just right, you can ride up the hand itself to avoid having to around. This is the case for both the one at the start and at the end.
3DS Piranha Plant Slide - When you first enter the underground area, use a Mushroom to cut the corner, skipping the path of water. A Mushroom is not needed, but it helps. In the Underwater section, it's possible to jump over the Pirahna Plant through two different methods. If you see a water current before the ramp, run into it jump over. If you see a water current right before the Piranha Plant, run into it jump right over.
Wii Grumble Volcano - The Mordor of Mario Kart. Use a Mushroom to cut through the rough terrain as soon as you enter the cave. A Mushroom can be used to cut through a turn on the right path at the end of the track.
N64 Rainbow Road - Shortcuts? What shortcuts? You mean falling and dying?! No shortcuts, especially not the long fall shortcut that lets you skip half the track from the original!