Fish tank decor,starting with gravel do not decorate with large rocks or marbles. This may cause your betta to get trapped or stuck between them. Remember bettas need to reach the surface to breath. So some pea size gravel will work great. Gravel comes in many different colors you can pick from plain,bright to black. Quick note pick the color gravel that will bring out your bettas color.
3/4 Gallon Betta Garden Bowl
Another good fish tank decor are fake or live plants. Fake plants come in all different sizes and colors. They do a good job of hiding filter lines,heaters or the plug in cords that show behind your tank. They also provide a place for bettas to hide if they get spooked or scared. While live plants do the same thing they also do much more. Live fish tank plants can improve the water quality of your freshwater tank. Live plants can remove harmful by-products like ammonia. One of the best plants for bettas is Java Moss. This plant is easy to grow, as mater of fact it grows like a weed. Do not let it take over your tank, just snip some off when it get to big. Java moss does not need to be planted it has no roots and need no sunlight to grow.
There many other Fish tank decors such as hollow logs, small ships and ceramic decors. Just remember not to fill your tank up to much with decorations. Your betta needs plenty room to swim around.