Nothing evokes the look of nature in an aquarium like driftwood. They introduce textures and shadows that provide visual interest. Branches stretching in multiple directions can pull an otherwise disparate aquascape together.
Note that some driftwood won't sink when initially submersed. Small pieces can be boil first to expedite sinking. Other let large pieces sit in water until they become waterlogged. Slate attached to the bottom of the can also be used.