Earlier this summer, the boys and I picked up a mess of PVC pipe and made marshmallow guns. Once you assemble the guns, you put a mini marshmallow in by the mouthpiece and then blow the marshmallow out. The marshmallow will negotiate all the turns in the gun and exit the muzzle - kind of cool really. The marshmallows melt in the rain, so clean-up isn't difficult either.
We got the initial instructions from Instructables. We used 1/2" (internal diameter) PVC pipe and cut the pipe into a few standard lengths (we used 3" and 7") for flexibility in recombining the pieces. Instead of the recommended hacksaw, we used a pipe cutter. This was easier and safer for the kids, didn't require a vice or bench, and produced cleaner cuts. This was the first time I've bought PVC; turns out you can only buy it in 10 foot lengths, so you'll have plenty. There was a hacksaw near where the PVC was in Home Depot so you can cut the long pieces down into something that fits in your car. (I got nervous when I saw the PVC initially...)
We didn't glue the pieces together (friction worked fine) so the boys were able to build all kinds of variants. They quickly learned that while the idea of a multi-barreled gun was attractive, dividing your limited lung power n-ways reduced the power.
The whole thing was super successful and very fun. We've since made water guns hooked up a hose and have been trying (unsuccessfully so far) to build a gun powered by compressed air (I've added a tire valve to a 2 liter bottle and pressured it with a bike pump.)
It's worth noting that I'm deadly with a four-foot section of straight pipe. I can hit the kids anywhere in the backyard with a marshmallow while sitting on the deck. Don't mess with Dad.
Of course, if it's worth doing, it's worth over-doing. Check out these crazy marshmallow guns...