Mario Circuit Goomba Boosts
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.
Rainbow Road Falling Stars
You may have noticed there are a lot of stars on Rainbow Road. And many of them fall. And some of those that fall fall on the track. Where those stars fall, they turn into star items, just sitting there. Some of these are dangerously close to the edge, but if you can get one (especially when you're already near the front and there's no other way to get one) it's a nice bonus. Only problem is that they're the same color as banana peels, so be sure before you run over something yellow!