I'm going to post some MK: DD shortcuts/secrets. If anyone knows any more please post!
Luigi Circuit: -In the beginning, there is a path to the right of the chain chomp with double item boxes at the end. -A bit later there is a small side path to the right that also has double item boxes
Peach Beach: -If you turn left into the pipe instead of right at the first curve, it will rocket you out into a double item box. -Also there is a shorcut in the beach section (on the left side) where you can use it if tide is low. -Near the end, where there is a Pianta statue, turn left around it for double item boxes.
Mushroom Bridge: -Right after the start line there is a small path to the right, which leads to a pipe with a double item box. -When you come out of the first tunnel, there is a hill straight ahead--use a mushroom to get over it. -When turning around the same hill, there is a stone pathon the very left; you go faster on it than on the road. -On the big bridge section, drive up the right side of the bridge for a boost/item box
Daisy Cruise: -When coming out of the main room with the tables, and after you turn right, there will be a hole in the floor on the next turn. Go into it for a double box and boost cannon thing.
Dino Dino Jungle: -When you enter the large area with the wood bridges, turn right immediately for a separate bridge with a boost. -When you enter the next big area (with the water fountains), turn right and use a mushroom to clear the gap to the island.