I just purchased some Pearl Weed (Hemianthus micranthemoides) for a future 5 gallon setup. It is in a 40 gallon tank for now. Hopefully, the stupid Kribensis won't nibble it to death. I think it would look awesome in a 5 gallon as a foreground or mid-ground plant.
Other plants that I think would work well for a 5 gallon and look good include:
Asian ambulia(Limpnophilia Sessiliflora) as a background plant as it get grow quite long and leggy, but looks nice if kept trimmed.
Riccia fluitans or crystal worth tied to rocks as described here and used in the foreground or as a boarder around stem plants or driftwood.
Ludwiga repens as a bacground plant
Rotala Indica as a background plant
Pygme Chain Sword or Echinodorus tenellus as a mid ground plant.
Crytocorne Wendtii(red to add some color).
And like I said 25% floating like water lettuce.
Any combination of these would work well. Plants you choose are really a matter of personal preference. These just reflect my personal preference for the 5 gallon I will be setting up.