A mirror for a couple of minutes a day will entertain your betta. They make floating ones but be careful the edges are sharp. I removed the mirror from the little ball it was attached to and put it on a suction cup.
Other betta toys I've heard people use but have had no luck with are ping pong balls and lasers (don't shine it at the fish, shine it into the substrate and they follow it like a cat).
I also have a moss ball in each of my betta aquariums. My female pushes it around and lays on it. My male just kind of looks at it but doesn't interact. He's more concerned with swimming around plants and flaring at himself.
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