A few good low light plants would be Java moss, and Java fern, Anacharis just to name a few.
If you buy plants locally watch out for SNAILS. They are nearly impossible to get rid of. Soak them for a few hours in a solution of ALUM and water, say about 2-3 tspns per gallon. Some people use a bleach solution (I forget the ratio) for a number of seconds. Do a google search for more info!
I never thought of my daughter as an "underwater gardener" until reading this article. She had a bad experience buying plants with snails that carried parasites. Ended up killing her beta fish. Now she buys underwater bulbs that grow beautiful plants. I'll pass on the tip about soaking the plants to rid them of snails. It's funny now thinking about all the houseplants that she's killed from lack of watering. Maybe that's what the aquarium is really about because I thought that she just really liked fish.