You could always do a water fall down your back wall if you'd like. You only need an area to hold your filter pump. It's all up to you and what'd you'd like best. Trial and error on some scrap foam and see what you like best.
Here's my favorite web site. Check out his waterfalls and backgrounds. You can create a rock or false bottom to hold the water rather than self-containing, if you'd like. Either way, these ideas can lead to more ideas.
http://www.lizard-landscapes.com/