How about this:
An aquarium inside a computer. And I don't mean some little aquarium window, I mean seal up the entire box, coat the motherboard and cards and everything so they are waterproof, and have the case actually filled with water. You would have to waterproof the hell out of everything that is submerged, and make a water tight case, and a few other things. you could use passive heatsinks on the NB and probably CPU, although you might want to locate a pump to flush some water across the cpu. You would also have to pump the water through a fairly big radiator to keep the water cool, and the fishies comfortable. Submerging the hard drives would be optional, but nice. Submerging the optical drives and PSU would be virtually imposable, and probably a bad idea.
Just image the sweetness of having fish swimming around in your computer.