I'd suppose if you wanted to you could build a waterfall scape into it with a pond at the base, using the back wall to build up on even extend it taller from the tank by fixing a piece of plexi to build on to the back wall pf the tank. After the rock scape plant it with small plants and mosses that will like the damp environment of the waterfall set up. It would just take a tiny pump and some tubing to move the water to the top from the bottom to keep it going.
Specific dimensions of the cube would be helpful as to whether you'd could keep something there once it's build but even if you can't, the plants can be rewarding also.
And that's I guess my point, a small tank can be scaped with plants and be just that. Submerged, emmersed or terrestrial. Up to you. Just in a small tank you gotta be particular and go for small plants too or it will look like an overgrown container.