Rocket Racers is a very fun activity that is designed for camps but can be done anywhere you choose. The basic premise is that campers build small cars that they then race against each other (safely) using model rocket engines.
This has many advantages over traditional model rockets such as:
1) It can be done inside or outside and isn't weather dependent
2) The whole activity is self contained so it can be done in a designated area (unlike rockets which can go everywhere)
3) It's more fun because campers are racing each other instead of just launching.
4) It's faster to set up
5) In my completely biased opinion, it's a lot cooler (this has been confirmed by campers and random staff walking by)
This Instructable shows how to make the Track that the cars race on, the Launching mechanism, the scoring Flags, and the pieces for the Racers as well as how the campers will put them together.
The Track is fairly simple, as is the Launching mechanism (based on a model rocket engine one modified to launch two engines at once). The scoring Flags were a bit difficult to come up with (if you want to make your own) but not very hard to make with instructions. The Racers are designed to be as easy as possible to make in bulk and very simple to assemble.
Lastly is how the entire system of the activity runs as well as safety instructions and concerns along with a bonus Engine Award mini-Instructable.