Heck, any of it would be fun with the proper messy factor! The ball looks like a blast - of the few videos I've seen, they put water and people inside. Water acts as sort of a lube so you don't get 'rug burn' by the plastic, I guess. I suppose gunge would be more fun in there, though the surface of the interior sphere would get all gunged, too, so suddenly it would be like being in a clothes dryer - you couldn't see out anymore.
You could put a few gallons of J-lube in there so it would still be transparent, then add a couple people...maybe naked!
