You have decor for two reasons: 1) To make the tank pretty and something you like having in your house, 2) To replicate the natural environment of your fish.
This means you have three things to keep in mind when you're decorating (aquascaping) your tank: 1) Will it make my betta feel safe, 2) Do I like how it looks, and 3) Will it hurt my fish?
Pshaw, Polly, I'm not just sticking random shit in my tank. I got it from the pet store! I don't need to worry about if it'll hurt my fish.
Bad news, Italicized Strawman. A lot of decor you can get at the pet store will hurt your fish. Some stores sell cheap (or overpriced) decor that has paint that'll flake off into the water. Also, betta fish have very delicate fins. Sharp or rough decor can rip right through them.
This is just the stuff that's not currently being usedLuckily, I have a lot of decor hanging around, so I can talk about the good and the bad.
The sand, gravel, or stones you put on the bottom of your tank is called substrate. There's several different kinds, all with good and bad sides.
Some people don't like gravel because it requires a more thorough vacuuming than sand does. You can't grow live plants in gravel, and it's dangerous for bottom feeders like catfish to be on. (side note, don't put gravel in your goldfish tank. They try to eat it and frequently choke).
The cup on the right in the above picture has glass stones. Large acrylic or glass stones are great for unfiltered tanks. They're easy to dump out, rinse off, and put back in the bowl. You don't need to worry about them going down your sink like you have to worry about gravel.
Some people use very large stones, 3"+, in their aquariums. This works really well for goldfish, but betta will try to swim through any little hole, and can get caught. If you use large stones, make sure they are smooth and that there are no spaces a fish could get stuck.
2.5gal tank with sandSand is another popular substrate. You can buy a bag of play sand at the hardware store for $4 for 20 pounds, which will be enough for most tanks.
When you're buying sand, make sure you're not getting marine sand. Marine sand has salt in it and isn't good for betta tanks. Play sand and pool filter sand both work for fish.
Sand is difficult to remove, and if you frequently rescape your tanks it is also very difficult to clean up and move. It's easier to bury plants in. You can't use it with a buried heater and you can't use it with an undergravel filter. It doesn't come in any fun colors.
Some people have an easier time cleaning sand than gravel, or feel they do a more thorough job cleaning it. A lot of people like that it's more natural looking. People who have tanks with live plants like sand for planting in it.
You can also choose to have no substrate and just have a bare tank floor. I like this because it makes it easy to tell if the tank is clean and very easy to vacuum, but it's harder to add more water without the decor sliding around.
It's also important to remember that if you're going to divide a tank, you need at least an inch of substrate to bury the bottom of the divider.
Now, other stuff!
DON'T TOUCH MY LEEF!!!!!Here's some betta-specific decor. The left is a betta log, and the right are betta leaf hammocks. If you have an older fish or a fish you've noticed sleeping on other things in the tank, it can be nice to give them a leaf that's meant specifically for sleeping on, or a log for sleeping in. Currently, none of my tanks have betta leafs or betta logs.
The top and bottom right ones have cups in the bottom that you have to bury to make it stay down. If you have sand, these stay better than if you have gravel. They do not work at all for bare tanks.
Now, on to plants:
Silk plants (which are made from fabric, but not necessarily silk) are my favorite for betta fish. They are soft and don't scratch delicate fins, and a lot of them have big leaves to sleep on.
If you want to test if a plant is good for your betta tank, put on some tights and rub the plant on your thigh (or pretend you did). If it snags your pantyhose, it's too sharp.
Well, that's more than anyone ever needed to know about fish tank decorations. Have fun, folks.