I'd suggest using crypts, especially Cryptocoryne wendti green, some mosses on woodden elements, Anubias barteri nana on stones or wood, also Microsorum windelow, or any tiny microsorum would be nice. If you like grass like plants you could also use Vallisneria tortuosa (the one with curly leaves) as the backround plant. These plants aren't demanding, grow nicely without co2 and in lower light conditions. Only for crypts and Valisneria it is good to put some clay-peat balls in the gravel next to the root system.
Aquariums and maths - two loves of my life.