If the Goombas on Mario Circuit are hit with a turtle shell, they will be squished into the ground and a boost will fly off them. If a thunderbolt is used on the course, it will squish them all and there will be several boosts lying in that area. It doesn't work the same as the mushroom cars; if you hit a goomba they stay solid and you spin around a bit. Turtle shells are you're best bet.