Toadette is in a rocky canyon. Pick the nearby plants for coins before dropping into the mine cart. There are several Piranha Plants here. The first gem is high up past the first set of Piranha Plants. Use the turnips to hit as many enemies and coins as you can. On the second loop around, you'll find the second gem under the track above. It's just before you reach the Para-Biddybuds. Use the turnips to hit the huge piles of coins and the POW Block in the center of the Para-Biddybuds. Nearby is another POW Block in some brick blocks. Hit it to reveal the third gem. Afterwards, you'll slide to the Star. There are some coins under the slide.
Special Challenge: Collect 140 coins.