You could also plant your plants in shallow plastic tubs. The small margarine tubs are good for this. Fill the tub with a good planting substrate, then bury the tub in your gravel. Put a few of the larger pebbles around it and you won't notice it. Some people plant in terra cotta pots and let the pots show, since they are kind of decorative.
Also you could plant on wood or rockwork, things like java fern, java moss & anubias. There are different varieties of java fern and anubias, so you could have lots of variation. These plants grow attached to surfaces, rather than planted, so the choice of substrate doesn't matter.