I saw a link some time ago where a chap had built a sort of 3D diorama behind his tank. Can't find it any longer but it was very cool. You need a bit of room behind the tank though. The guy was a bit of an artist and airbrushed a great graphic, added a few fake rocks to make it look 3 dimensional and also had some subdued lighting above the scene he created to highlight the features. It actually looked a little eerie and of course you'll need to keep the back glass spotless. You don't need to be an artist like he was, any good quality poster will do with the theme of your choice. If I can find the link again, I'll check back in and post it.
Ok, this isn't the one I saw, but it gives you the idea & shows how to make one.
http://www.glaucus.org.uk/CB1.htm