The key is in the Ancient Lake course. As soon as you start turn onto the grass . You should see a small ramp. The key is at the top of the ramp.
SNOWFLAKE MOUNTAIN
The key is in the Snowflake Valley course. Right from the start turn off the tack to the left. Follow the little path a little ways. The key will be on your right.
SHERBERT ISLAND
The key is in the Crescent Island course. The hovercraft is your best choice to get this key. At the start of the course you have the choice of going right or left. Go left and hit the zipper and go out into the water. You should see an opening in the rock on your right. The key will be in the opening.
DRAGON FOREST
The key is in the Boulder Canyon course. Go to the wooden bridge leading into the castle. On the far side of the bridge there is a bell with a blue ballon in front of it. Get the ballon and wait for it to reappear and get it two more times so that you have a level 3 boost. Drive under the bell and trun around so that you are facing back across the bridge. The bell should ring and the bridge should start to raise. When it is up use your turbo boost to shoot up the bridge. You should land on a ledge with the key on it.