Make slate caves . Glue the slate pieces together with aquarium sillicone, leave overnight to cure, then put them into the tank. Tie java moss or Java fern to them where you can to make them look less obvious and more natural.
You could also do this with other types of rock. Just make sure they are well attatched and stable when you place them in the tank so they don't topple or fall apart and crush anything .
Tall plants are also good. Cabomba and elodia are easy and good plants to use. Floating plants like Riccia or Frogbit is good too.