I have a 1-gallon cheapie tank at work with an undergravel filter in it; my boss gave it to me 'cause all the fish she put in it kept dying.
I set it up with tiny natural gravel from my 55, a couple stones, a java fern and some moss, and the tiny light on a timer. Boy, does it drive my co-workers crazy. First "why is it EMPTY"? Then you get to explain the nitrogen cycle. Then "where are the FISH" when all I have in there is two physa (aka 'pest') snails that I pulled out of my snail tanks at home. Then I put an MTS in there. Then a baby apple snail. Then the physa snails laid eggs, then the eggs hatched, then the MTS put out a couple babies, then.....
They are very excited that I will be adding a couple H. formosa next week, and many of them can now ID bridgesii , MT, and physa snails on sight!
It gives you a good opening to educate people on why there isn't a nice goldfish in the tank. My boss asked where the fish was...I asked her where her fish were, the former occupants of this little tank. So she got to learn about the nitrogen cycle.
It's fun to have people stop by and see what's going on in this microworld. Maybe keeps a few goldfish and bettas alive, too, when people learn what needs to be done to care for them properly.