I'd say a betta and maybe a few rasboras, you are really limited! I believe you could grow some anubias nana and java fern probably. If not you could get a desk lamp and shine it in using a 13W or so CFL bulb which would help alot. I love these little tanks. I definitelly wanna see some pictures.
Okay, so since i was all over the place I'll give you a step by step of what i would do.:
1.dark dirt or sand substrate.
2. Nice piece of drift wood
3. Possible lighting upgrade if needed for plants.
4. Betta for sure, possibly rasboras
5. Also plant anubias nana, java fern, maybe a moss ball in the back corner, or even open and spread it over the drift wood. (get other plants if lighting was upgraded such as amazon swords etc.)
6. If there is room maybe put some small to medium sized rocks, other wise leave it!
Please upload pictures and happy scaping to you!