Fun fly sticks
They produce a fair voltage and can be used for lots of electrostatic repulsion and attraction experiments, you can even get small sparks out of them.
This is pretty impressive, but they are also the most reliable van-der-graaf I have ever come across. Van-der-graafs are renouned for not working if the weather isn't right or it moon is in the wrong phase, but the fun fly sticks just seem to work. I think a lot of it is that they are entirely enclosed and filled with silica gel.
I was so impressed with them that I thought you could do some great exhibits using them, and they are dirt cheap, so if they don't last forever it wouldn't be the end of the world, but do they have sufficient life to work in a science centre?
To find out I rigged up one to a power supply and just set it running, and running. It lasted 2 days fine, but by the next morning the extremely cheap motor they come with had failed... which was sad. and even if it is only running when a button is pressed this isn't going to be long enough.
I may try rigging up one with a better quality motor and see how long the rest of the mechanics lasts...