Here is my tank...
Both of the decorations I got from Petsmart. For the cave on the right, I liked it because the inside
of the decoration was smooth. Oftentimes the inside of the openings have jagged edges that can cut your betta's fins. The dangling shapes is actually a bird toy that I took the metal off of and rinsed in warm water. I switch it out with some diving toys every once in a while. You can also use dishwasher-safe coffee mugs or mason jars that have been rinsed well in warm water.
Edit: Also, the plastic plant on the left is a popular plant for bettas--they can swim through it/rest on it. I've caught Iv in it a few times. :)