I love the natural look so I make my tanks full of plants and driftwood.
My favorite plants are dwarf hairgrass, java moss, marimo moss balls, lace & regular java fern, anubias nana petit, amazon swords, money wort, water sprite, rotala, crypts, jungle val, anarchis...I'll just say that I like pretty much all plants as long as their growth rate is moderate to fast and they do well in low light conditions. :)
I have gravel right now, but really like play sand or tan "natural" looking gravel for substrate