I put a large floating glass bead, Elmo loved pushing it about. Ping pong balls scare him, he hides in his cave until I take it out!
I also made a temporary hoop for him to swim through from a pink pipe cleaner, which sometimes he'll think "this is amazing, I'm swimming through it, I'm clever!" Other days, he'll look at it, and have this blank look on his face!
My other betta - Oscar used to pace up and down one side of the tank. I then realised he was bored, so I used to put bright coloured items outside his tank for him to 'inspect'!
I do the same with Elmo's tank. Whenever we get junk mail through the post, like a Domino's menu, or Pizza Hut menu, I put it infront of their tanks, and they 'read', maybe looking to get some take-out!!!