We just did a race car birthday in December for our 5 year old. I like to do crafty things so a lot of these ideas are do it yourselfers. First off, if you go to familyfun.com you can see the cute race car cake and game ideas. I did one of their game ideas- Each child took a turn with this. We took our Volvo and I made sponser signs with each child's last name to decorate it. We made a relay race of sorts. They had to "pump" up the spare tire using my husband's air compressor, then run to a small step stool and clean the window with a washer, fill up a bucket next to the gas tank to mimick gas, hop in the car, put on a helmet and honk the horn. At first we started to time them but they just liked doing it so we stopped doing that. I also had them paint wooden cars that I found at Oriental Trading. Another idea which I had ready but didn't need to do was using the foam "paper" you can get at Michael's have them create personal license plates. When each child came in I took their picture with a poloriod and wrote pit crew on the bottom of it and tied it a string around their neck (I borrowed the camera but the film was one of the most expensive things). As a favor I bought the small model cars at Michael's in their dollar section. If you look for their wooden things to paint you should find them there- they are race cars and come with EVERYTHING you need to put it together for $1!!! They are perfect. I also bought black, red and white paper stuff from Oriental Trading. For lunch they at the concession stand and had hot dogs, applesauce, grapes and strawberries, jucie and hot wheels fruit snacks. I had salad for the adults. You can also find candy hot wheels at Oriental Trading. It was a fun birhtday and I had lots of compliments.
Best of luck!!
Oh- I also wore my husband's coveralls he uses when changing oil. I think you can buy them used for cheap at a store like pep boys.