Java fern and java moss can be grown attached to driftwood or other decorations, or even floating. Frogbit, dwarf water lettauce, and water wisteria are also good floating plants and I believe pretty easy to care for.
Amazon swords and achrchnis (sp?) are also good low light plants and I think would survive in regular gravel.
Make sure you have proper lighting for plants and dosing with a fert like Flourish or even adding Excel (but take your moss ball out when you do excel) will really help your plants.