Anubias, Java fern, bolbitus and mosses can be grown attached to drift wood or rocks.
Some of the easy to find and grow stem plants are Rotala rutundifolia, Hygrophila polysperma, Ludwigia repens, ambulia and star grass.
Crypts and swords should do ok in gravel as well, but since these are heavy root feeders they will much prefer a richer substrate.