There is nothing better than live plants. Even the best silk plants don't compare. The best and easiest plants to start out with are Java Fern and Java Moss. They grow in low light conditions and are quite hardy plants. Neither needs to be planted in the substrate. The Java Fern can be kept potted in the basket (if it came in one) or gently tied with cotton thread to a piece of
driftwood, ornament or rock. The roots will eventually wrap around and attach themselves to it. Both plants are excellent at removing nitrites and nitrates from the water so yes they do change the water, for the BETTER.
Java Moss is a tangled clump of mossy looking strings with no roots. It just sinks to the bottom or can be tied to anything you want to give a nice appearance to the tank. It is very soft and my Bettas LOVE to sleep on it or burrow under it and cover themselves with it like a blanket. It's really fun watching them play with Java Moss. The Moss grows very fast and may need to be trimmed back once in a while or clumps can be transferrred to other tanks or given to friends with tanks. Java Moss should be kept away from the filter flow though or else the turbulence will send little pieces flying all over and they can get in your filter. I just keep them on the opposite side (still side) of the tank and I've had no problems at all. It stays pretty much all tangled up like a ball of soft wool.
Java Fern grows pretty fast too. The potted Java Fern I bought a while ago has become such a huge bush I've taken many chunks of it and distributed to other tanks.
They require nothing more than a little light (Java Fern and Moss actually do best in low light which is what we all use to light the tank) and the vitamins you add to the water for the fish will also benefit the plants.
I got my plants at Big Al's. They have the best plants I've ever seen and the plant tanks are so clean you could drink from them! You probably don't have a Big Al's near you (too bad, I wish everyone here had a great store like that nearby) but you could order plants online. I've never ordered online so maybe someone here can direct you to a good source. I would stay away from Petsmart or any or those types of stores for plants! They have terrible sickly plants that are full of snails.
I'd recommend a smooth polished river rock about pea sized gravel. It looks very natural and is just the right size to trap dirt deep beneath the gravel and away from his fins and can be easily vacuumed each week. That's all I use on my Bettas and they've never had finrot or torn fin problems from it. Somes types of gravel can be rough but as long as you make sure you get the polished river rock or a coated colored gravel, like the type made by Marina, it will never hurt his fins.