Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
In that small a tank I could suggest things like Dwarf Anubias, Wendelov Java Fern, Mosses (one of the more compact ones, not the stringy Java Moss). Crypts are likely to stay smaller under low light conditions, so if that is the way you are thinking, go for it.
The lowest light ground cover plant is probably dwarf Clover, Marsilea.