If it were me, I'd go one of two routes:
- Some easy stem plants like Bacopa and Ludwigia in the center that get pretty tall with no hardscape. Go with Cherry Shrimp and snails.
- Get a nice branch of Manzanita or other spindly driftwood and attach a bunch of moss to it, put Anubias nana 'Petite' at the bottom. Cherries and snails again.
It appears to be large enough that you could hide a small sponge filter in there with ease.