What type of lighting will you be using? If you can find an incandescent strip light for your 5g, you could use compact fluorescent spirals (look for 'daylight' bulbs), which would give you enough light to grow many types of plants.
Anubias is a good suggestion, not only is it tolerant of lower light, but it grows fairly slowly so it won't take over a small tank.
You could try using moss as a carpet - attach it to a wide, flat rock, lay the rock down at the front of the tank, keep it trimmed and voila! People do this with riccia, but if your tank is low light Java moss might be a better idea.
Cryptocoryne wendtii might make an interesting background plant in a small tank (would look something like swords used in the background of a large tank) - it will grow to somewhere between 5" - 8" high (there are different varieties, and size might vary depending on conditions).