Quite a few things you could keep in there... there's a list of potential nano fish here - http

/www.plantedtank.net/forums/planted-nano-tanks/31095-nanofish-list-v1-0-a-7.html#post904496, obviously you'll want to research further for suitability but might give you some ideas.
Personal recommendations would be things like one of the many microrasbora/boraras species, sparkling gouramis, licorice gouramis, pygmy corys, small betta species, badis, pygmy sunfish. Basically things which stay small and don't require too much swimming room.
Depends a bit on the footprint of the tank and obviously your water paramaters and whether you can meet the needs of individual species, some of them can be a bit dainty. I think single species tanks work best for this size aquaria too.
DPs are great though