It's quite easy to build a launcher for pressurizing 2-liter bottles and releasing them.
Have a contest for building a rocket that flies the highest. Materials are CHEAP! - empty 2-liter bottles, some posterboard, cardboard, duct tape. The "engines" for the rockets are one 2-liter bottle that is filled part way with water, and pressurized (most people limit it to 90psi or 100psi for safety's sake). Students can design their own fins, and decide whether to build a short stubby rocket (just the "engine" plus fins and a nosecone) or a taller rocket (up to 2 or 3 2'liter bottles tall.)
Scientific process: determining the "ideal" amount of water to use. 1/4 full? 1/2 full? etc. This can be done by experimenting.
Also, which works better? Taller or shorter rockets? Taller rockets will naturally be heavier. (But, not be as affected by air resistance)
How to make the fins? Big long fins? very short fins? What's sufficient?
You can limit the parameters that the kids are allowed to vary.
Materials necessary for a launcher:
bike pump
inflator piece from an old bike tube
1/2 inch cpvc (one full piece, which will be cut a bit)
3/4 (or is it 5/8?) CPVC - about 4 inches - should barely fit through a 2-liter bottle neck
o-ring (that will fit over the outside of the 1/2 inch cpvc)
and a few cpvc connections - a t, and a cap, plus optional: a couple of corners
cpvc cement
piece of wood (2 foot 2x6 works great) to mount the launching end to.
A couple of pipe straps (to secure it)
2 metal L brackets, and a nail with a string on it (for holding the neck of the bottle down)
The most expensive, but optional part, is a pressure regulator to guarantee everyone has the same pressure.
If you want more details, how to build the launcher instructions, etc., there's a ton of info on the net, or you can PM me.
I do a similar experiment with my high school physics kids. A lot of them get really involved with it. The only difference is that they have to launch a raw egg, keep it in the air as long as possible, and keep it from breaking when it returns to the ground (catching not allowed.)
It's amazing how difficult it is for them to design a parachute system that actually deploys.
If you *really* want to push the elementary kids, you can teach them some simple trig for measuring the height of the rockets, or you could leave that up to the judges. You can also videotape the launches from a stationary (on a tripod) camera some distance away... slo-moing through the launch and marking a place on the television screen that represents the highest point should alleviate most of the "not fair!" concerns. Or, a simple "whose rocket is in the air the longest" would be well within their reach... the kids can use stopwatches to make sure they agree with the official judges' times.