Mario Kart: Double Dash!!

As you may have noticed on the Mushroom City course, among the traffic are vehicles shaped/colored like the Mushroom items. If you run into them, you don't spin out like you would against other cars; rather a Mushroom is thrown from the back for you to collect/use for a quick boost!

In the balloon battle mode on the Pipe Plaza map. if an enemy hits you into a pipe with their weapon, and you are thrown through the pipe then you will not lose a balloon. Instead, you will come out of the pipe with stars around you and it will appear as though you just drove through the pipe.

To unlock everything, just win Gold on every Cup in every difficulty.

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!