Yes, I agree, a betta is pretty much the only fish you could put in there. You could put a snail in with the betta. Nerite snails are good because they will eat all the algae and they can only reproduce in brackish water, so your tank won't be overrun with snails. You could also put in some shrimp with him, but this doesn't always work out because the betta will sometimes eat them. You could even make this an all shrimp tank.
As for the scaping, you could do an asian theme if you decide on a betta. You could get a little temple and a buda. My betta tank has some clay pots in it. You just have to make sure to sand down any sharp edges and cover the hole at the bottom.
There are so many options when it comes to plants. Anacharis is a great plant in my opinion. It grows really tall really fast, and you have to work really hard to kill it.
Anubias is a really nice plant and it comes in all sorts of varieties. The only thing about the anubias is that it is a low light plant and you can't bury the roots in the substrate.
I also really love marimo moss balls. They look really cool and unique, and fish, especially betta fish, love them. They are also really good for your water because they take some of the nitrates out.
If you want to carpet the floor of your aquarium, you could use dwarf hair grass. I haven't had any experience with this plant so I don't know much about it, but I do know that it doesn't need any fertilizer or much care.
The type of substrate you use depends on what plants you are going to have. I just have gravel in my tanks, but you could get special gravel for plants, you could get sand, or those colorful plastic gems. If you have plants like the ones I mentioned above, you don't need any special substrate. I have all those plants except for the dwarf hair grass, and they are thriving without any fertilizer or special substrate.
Good luck!