A nice animal to use in a bottle with water! Fill the bottle with water and put the jellyfish in it.
Gently squeeze the bottle with both hands and the jellyfish will drop down. If you let go of the bottle, she swims back up. If you can do this well, you can try squeezing the bottle firmly and less firmly, in a fast rhythm. The jellyfish then starts spinning in circles. This requires dexterity from children. If the jellyfish no longer works, take it out of the bottle and blow it dry. There are two small holes in its head. By blowing on one of the holes, you can blow the water right out. Let it dry a little extra and then the jellyfish can go back to work.
A PET bottle is most suitable for this.