You can get silk/material fake plants that are soft and ok for bettas. Why not try real ones? They aren't too hard if you pick low maintenence species. Aponogentons would be ok, Cabomba is very easy to grow and has very soft fine leaves that are perfect for a betta. Java ferns and Java moss grow fine in low light too. Some Floating plants like some Salvinia natans can also look good. I have all of the above plants in my tank ( except java fern ) and my mothers betta loves them.
As for other inhabitants, I have mentioned all the ones I would put in there myself. I already have a 12 gallon with a Dwarf gourami and pygmy Cories in it ( second pic in my sig....just a bit of it anyway haha! ). It works very well indeed . As others have said, if you wanted something slightly unusual, then a dwarf puffer would work in there, but as far as I know they like to live alone.