Bhoomplay has a video tutorial for how to make a jellyfish in a bottle and they look so real. All it takes is a plastic water bottle, a transparent plastic grocery bag (the thinner the better, like the ones used in the produce department) and some food coloring.
Good for ages 6 and up.
Get the instructions here:
http://bhoomplay.wordpress.com/2011/09/09/diy_jellyfish_eng/