The forest rivers, ponds and lakes often have a lot of wood in the water that provide cover and spawning surfaces for fish. Cured wood can be bought from your local aquarium store that will sink to the bottom as soon as you put it in the tank. Don't be alarmed if it turns your water yellow (known as tannin), this is normal. The tannins from natural wood that turn the water a tea color are often useful in triggering some soft water fish to spawn. You can remove tannins from the water by adding a good grade activated carbon to the filtration system.
Coconut shells can be used to provide shelter and spawning sites for many species of fish. The density of the coconut shell allows the shell to sink to the bottom of the aquarium.
Coconut shells can be used to provide a spawning site for many species of cichlids. Wood in the background gives this aquarium a tropical rainforest river look.
Some of the wood sold in aquarium and pet shops are coated with a plastic type resin to keep it from turning the water yellow. I do not recommend this type of wood, because there are many species of sucker mouth catfish like clown pleco (Panaque maccus) that enjoy chewing on wood.
Drift would can also be collected and used in the aquarium. Be aware that not all wood is safe for the aquarium. Some wood has natural toxins that can become toxic for fish in a closed system.