This discussion has got me wonderign about adding jellyfish, as they would certainly add some height to the game.

These are silicone aquarium ornaments. Very cheap on Ebay, but far too large - the smaller ones have a 5cm wide canopy and are about 15cm long. They are designed to float in water, so I'm not sure how well the canopies would hang in air.
As for scratch building jellies, the problem with them is that they are translucent. Solid jellies just wouldn't look right.