Those of use who know from experience can safely say there's nothing quite as much fun as a homemade rocket. But part of that fun is not getting your hands blown off. Nighthawkinlight has a fun, easy way you can get the best of both worlds, and "recycle" your old water bottles in a literal blaze of glory.
The principle is one you'll be familiar with if you've ever built a potato gun. The spark from a modified grill igniter lights up the alcohol vapor to create an explosion, except in this case, you're firing the ignition chamber itself instead of launching a tuber. There's not much risk involved, other than the possibility of a tiny little flash-burn, and any good pyro needs a little variety when playing with fire. [Nighthawkinlight]