Find a decoration (or plant), you really love, and go from that...It can be the center piece (like those room decorating shows talk about), well...when the betta isn't impressing with it's beauty!
Backgrounds can be anything from laminated premade ones, to your favorite work of art printed out.
I like dark gravel personally...It looks cleaner & brings out the fishes coloring (Unless you have a black betta lol)...or natural colored gravel, or sand...
There are also many imitation coral/live rock decorations, you could do faux salt water theme...
Lots of "tiki" items as well...palm trees & huts.
Easter Island heads.
Endless possibilities !!!