They will outgrow that tank, but to answer your question --- if it is a display tank, some wood, rocks, caves would be nice. Plants won't do well, b/c Jack Dempsey's are diggers and they'll mess up any plants you try to plant. If you like the visual of plants, then I'd recommend getting some of that tank backing that you can tape to the outside of the back of the tank.
If you are looking to spawn them, then put in more caves.
Don't forget those water changes. To keep them healthy in that tank, you'll probably need to have water changes at least once a week.