Mostly cories, some clown loaches, dwarf neon rainbows, rams, apistos...
When I bought him, my DP was in a 29g with about 30 others and a male guppy. Completely bare aside from the gravel. No one had nipped fins, they were all just kind of floating around looking at the surfaces. At home he's in a 15g with 3 black kuhlis and a nice male guppy, no signs of any agression... So on that note, I say if you can find peaceful dwarf puffers, get a couple dozen of them.
You could check out the International Aquascaping Contest, they have different categories depending on the size of your tank. Some of the smaller ones look really great I think.
I personally would go with a completely non-aggressive tank, and as far as smaller fish go, you could get huge schools. Bottom feeders, top feeders, middle feeders. I bet a whole bunch of bright (female) honey gouramis would look amazing.
I would probably leave the water level somewhat below the rim, so I could have some floating plants and plants above the waterline. Then I'd add small fish . Marbled hatchets, glowlight or rummynose tetras, lemon or serpae tetras, otos, cories, a dwarf cichlid species, bulldog plecos.
No big fish for me. I always start to think of what kind of sauce I'd like with them when I see them .