Finally, I settled on the tornado in a bottle. I had already printed off instructions from another site for making one of these anyway, because I had two pop bottles under my sink, and I wanted to use them for something. Basically to make the tornado, you need to fill one of the bottles part way, and then connect the two bottles together at their mouths. I've found quite a few suggestions for doing this, from poking a hole through the caps, and then gluing them together with water proof glue, to placing a one inch washer between the open bottles, and taping them together.
Of course, you can buy a tornado cap from almost any educational type science store. The toy maker site, suggests connecting the bottles together by sliding them onto a small piece of 1/2 PVC pipe, with a piece of duct take wrapped around the middle of it, then sealing the bottles with plastic wrap, and taping over that. They also give instructions on how to get an even air pressure in the bottles, so that the bottles won't dent when you use them, and explain the science behind the vortex you're trying to create.
I think this project is supposed to be suitable for older children, but I made ours myself, and then let the kids use it to make the tornadoes (you give the bottle a good circular swish after you turn it water side up). Even so, I found it to be a little bit tricky to get the seal tight, and ended up dousing our kitchen with water before finally succeeding.
If you don't have two pop bottles, you can follow the instructions for making a tornado in a jar at www.weatherwizkids.com/tornado2.htm instead. This is also a great website to check out it your thinking about doing a weather unit with the kids.
It's great to be a homeschooler!