Bhoomplay's Blog has a pretty cool little tutorial on how to make a jellyfish in a bottle. Ok, it's a plastic bag, but it looks really cool.
Check out how to make this here. There's even a cute video showing kids playing with them. As soon as I can find someplace that has clear plastic bags, we're going to try this out. All my plastic bags are white or brown. I don't think it would have the same effect.