If you are still deciding on tank decor here are things I think about
1.) what room is it going in and do you want it to coordinate at all? (Not going overboard, but for example someone I know has an Asian themed room so I got their betta tank a little red bridge and the asian bridge background. That's all I did, the rest are all green plants and soil)
2.) I think you already said you're doing black gravel so I assume your fish is not very dark and would stand out nicely. I would always do this - I'd always contrast the fish to the decor .. although now I only have a planted tank so everything is just green.
3.) Just make sure you get a hidey hole thing.
4.) If you get driftwood and you have a working light, you might as well tie a java fern or anubia to the driftwood. It's low maintenance, low light, and helps the water quality. And the java fern will grow.
What color is the fish you're making this tank for? I missed it.
Here are some progressions of my old 5 gallon I no longer have. Now I am just doing boring old plants in a 6.6 gallon.
I added 2 photos of some 6.6 gallons I used to have just for color schemes to look at.