Delfino Square Shortcut:

Go through the city like normal after you cross the bridge turn left and go strait. On the right there should be two boxes. You may think these boxes are just obsticles but they are actually blocking the entrance to a shortcut. Go around them and there's an alley. When you go through the alley it is full of mud that slows down the kart. Make sure you have a mushroom to speed up with so you keep at a good speed when you hit the mud.