Put a head or some floating brains in it. Run some EL lighting wire to create a life support system. Or, put an arm bone, add some flesh to it and then attach Piranha fish and run air out where they are feeding.
It was nothing really fancy but several years ago, I made a sculpey brain for a party and put it in my old tank, along with a doll eye (would've used two, but I'd misplaced the other one). Added some highlighter dye to the water and put a blacklight in the fixture.
The brain had two screws in the "temples", to which copper wire was attached (there were also short bits in the end of the medulla oblongata). The wire attached to a thin wooden bar placed under the light fixture, suspending the brain midway in the tank. Air trapped in the doll eye and the natural rigidity of the tubing made the eye tend to float. A long air stone was placed in the bottom to provide "oxygen" (and the circulating water from that and the filter in back made the eye tend to bob around randomly a bit)
Nowadays I'd do something more elaborate, possibly using 3d printed parts (i.e. trim on the tank, mad scientist widgets, etc.) I still have the brain and eyes, but I'd have to get a new tank (or use one of those giant cheese puff containers like some people do)