Definitely avoid the starfish. A live one needs salt water, a dead one will just fall apart in your tank.
The flowerpots are a good idea. You can also get resin decorations that are like small castles, logs, or caves that work great. You could also buy some aquarium sealant and glue together some slate to make caves. There are other threads that will be able to tell you how to treat any rocks you want to use in your aquarium. Also, driftwood and live (or silk) plants can provide good cover for a betta. Avoid plastic plants though, as they can easily tear a betta's delicate fins.