Sunshine Airport - Am I the only one that thinks of that great Airport map from Modern Warfare when I play this? Just me? Anyway, the only shortcut in this map is once you come back into the lobby, ride through and perform a trick through the airport conveyer belt to cut through that corner!
Dolphin Shoals - This a really great looking map with only one secret that requires a mushroom! Just before you reach the end of the track, use a Mushroom to cut through the rough terrain on the right to pass over the last turn.
Electrodome - So this is how characters from Mario Kart go partying apparently. They are literally dancing on the ceiling, Lionel Richie would love this!! This place has two tricks, at the start of the track, use a Mushroom to cut through the dark terrain on the left. Secondly, at the end of the track, use a Mushroom to cut behind the Piranha Plants and to pass through this corner.
Mount Wario - This is one long continuous ride down a mountain, no laps here! Before you enter the icy cave, jump off the small ramp on your right just before the tight turn. You can see this in the picture below. When you reach the forest area, use a Mushroom to reach the gliding section in between the fork in the road and also use a mushroom to cut through the very last turn on the corner.