Do you mean like java moss? Or something more leafy? I had java moss in a tank of mine before, it was very good looking and quite hardy, unfortunately it didn't spread at all. The ghost shrimp I had in the tank loved it, it was so neat to watch them crawl around in it and pick at it with their little pinchers...
Speaking of shrimp reminds me, are you going to have any kind of creature in the freshwater aquarium that's equivalent to the starfish in the saltwater one? Like a snail or something? I know you said frogs and crabs were (understandably) out until a creature pack, but a nice big apple snail would be cool and comparatively easy.