Mostly because I've been really looking into them lately, but I'd suggest a puffer for a unique oddball fish - several of the FW ones stay small enough, or else mixing salt to get brackish is not as hard as it seems before trying it.
Or, another of my favorites is a Senegal bichir. Or an African leaf fish, or African brown knife.
All these should stay small enough (if done singly in the case of the puffer/bichir, or a combo of say leaf fish/brown knife for a long while) to stay in that size tank.
If you want to go small cichlids, maybe a pair of apistos for the bottom, then a few tetras (I like black neons, lemons, and rummynose) for middle, and a couple hatchets for the top. Or golden dwarf cichlids - I've got a few I've been growing out I got indirectly from GrizzlysDad (via a raffle at the last auction), and they're quite nice, especially with a tan or off white substrate and plants.
If you wanted interesting behavior from some African cichlids, gold ocellatus or multifasciatus are fun to watch digging...but a 20 long is just fine for them and the top 6-8 inches will be pretty bare.
These are a few of the 'less common' suggestions you might get. You could always go with old standbys like German rams, or a trio of yellow labs (at least while young), etc.