Real-life Mario Kart Koopa Shells are always a good idea.
Toy designer Skimbal has created a do-it-yourself kit that allows you to create your own Koopa shells from the greatest racing game ever in the history of the world: Mario Kart. Michael Curry, known for his work with the Gothic Cathedral Play Set (for those of us with children), was previously laid off from his job at an architecture firm and he decided to make the most of his time by making our lives that much better. The shells were created with his $700 MakerBot printer.
After Curry designed the Koopa shells and put them online, the owner of MakerBot was so impressed with the job Curry did he gave him a dream job at their studios.
The free-of-charge kit is available online from a number of sites. The only requirement is that for the best-looking shell you can find you need a 3D printer. Print out the schematics, attach them to your R/C car and there you go: Mario Kart Koopa shells. Several different versions are available, including the shells with wings and four and six spike variants.