A themed tank might be fun - somebody mentioned the khulis... maybe an environment designed around a specific fish(es). Maybe lots of dwarf cats/bottom feeders in a maze of caves/stumps/plants. Maybe tubes burried under gravel so the fish can swim around the tank when they wish - perhaps clear tubes so you can see them?
Or large schools of small fish - like guppies, neons, etc. and.. could you put mirrors in the tank? It would reflect off their numbers and make it look like a ton... don't know if it would freak the fish or not...
Good luck!