Mario Kart Dimensions features three main collectible items that can be found on the track, those include Morph Bubbles, Coins and, the most common, Item Boxes (or Item Blocks in Europe and Japan). There is a fourth collectible, and that is a "license".
A coin is a common item which is represented by a large golden coin that will be floating a short way above the track and will spin. The coin will, when collected, will make the racer move quicker, just like in Super Mario Kart, Super Circuit and 7. The coin is golden (which gives you 1 tiny boost of speed) and a silver coin (which gives you 0.5 boost of speed).
An item block/item box is the most common item that is represented by a semi-transparent, rainbow-tinted ? Block. The ? Block will be spinning around on the spot and will be "destroyed" when touched. The block will then respawn 1 second after.
Morph Bubbles are special bubbles that appear which turn the racers into submarines to race underwater. Morph Bubbles are rare, as they only appear on minimal amounts of courses.