My betta thinks everything that goes into his tank is food.
My hand when I'm fixing something in his tank or doing a water change, bubbles from the filter, anything he catches moving.. he will nip at it repeatedly. So toys that I've tried have been pretty unsuccessful. If I try to float a ping pong ball or a bottle cap, he just nips at it, thinking it's some sort of food.
I sometimes light a tea candle and set it by his tank, he's usually pretty intrigued by it. I've set open books against the glass, and he takes hours inspecting them.