Corydoras. IMO, no community is complete without a half dozen or so of these guys on the gravel. Great personality, and if you have 5-6+ they're very active and playful.
I'd also add something that sticks towards the top of the tank, like hatchetfish or danios, to get some activity at all levels. Pearl danios are my favorite danio by far. Don't be fooled by the pale pink they show at the stores usually, once settled in they're a blaze of red and blues.