Plastic plumbing pipe is easily worked and can be used for all sorts of things.
plastic pvc plumbing pipe flute described how to make a long lasting musical instrument. The page also has good advice for working with PVC in terms of safety and tools.
Adding a gray water tank is another example of a big pipe solving a practical need.
Flagpoles are another possibility. Normally, plastic pipe is used just as a sleeve in a ground mount for a steel or aluminum flag pole because plastic pipe isn’t really stiff enough for a solid and tall flag pole. But, if you are into demonstrations with flags, Jozef’s Flag Pole Recipe – How to make a Veterand For Pieace Portable Flagpole has detailed instructions on building a plastic pipe flagpole. Of course, if you aren’t making a bit hole with concrete and a plastic pipe sleeve to hold your flag pole upright, you will need something like the hitch mount or RV stand at Banner Marketing or the ladder mount at Camping World. An idea for a homebrew version is in Home & Garden Television: Entertaining:
The pole and base can be made by securing a three-inch flange to a piece of 2-by-8-by-18-inch plank. Attach a 36-inch-long, three-inch diameter PVC pipe to the flange. The PVC flagpole can stand tall in this base because the base plank is anchored under the tire of the car.
Another popular use for plastic pipe is as a holder for the drain hose. A 4 inch drain pipe can be mounted underneath the rig near the axles can provide convenient out of the way storage.
Of course, you could also use plastic pipe for plumbing, but that is another episode!