This week is Diddy Kong racing.
Yes it is kind of childish but it is actually quite challenging. Not the first level, it is really easy. Wait till you get to the secret outer space level. Yeah, secret outer space level. You have to beat every thing all the races, all the silver coin challenges, and all the trophy challenges. Then you have to go to the sign boardrun right in to it.

Here's a little tip
In the first level (Dino Domain). Grab a missle just before the dinosaurs and have a little fun and shoot the dinosaur. Not only is it fun but if you hit it in the right spot it will run in front of the other racers. This doesnt always work though.