Below is a picture of a simple race car track my 3 year old had a blast playing with at a local family fun event.
The cool thing about a race car ramp/track is that you can through some simple physics into it and experiment with different ramp heights, angles, and loop sizes. For example, the first loop that was set up didn't work very well. The cars weren't making it around the loop. The curvature of the loop was too tight and the car was getting stuck. A simple adjustment and we had an awesome loop that worked! You can also test various cars to see which have more friction in the wheels. Got a car that doesn't make it to the loop, but others do? Friction is the culprit!
My 3 year old had a blast with this activity and didn't want to leave!