You could try turbo snails but get small ones, they're less likely to be eaten than the larger ones. You could try cleaning magnets, some octos love to play with them, they're like toys. Cleaning a tank is also a time where interaction can take place. Try those sponge on the stick things ( i forgot what its called) and the two of you might end up playing tug of war, but this depends on the octo really.
Many of our Tonmo octos have enjoyed playing with cleaning magnets. My bimac Ollie loved to pull off the in-the-tank part and hide it, dangling it out on the tip of one arm so we could try to get it from her. It was also intresting to watch her take it into her den, because she understood that she had to turn it just so to make it fit.
Later she used the magnet when she wanted attention. The big thump of the outer part falling could be heard anywhere in the house, and always someone came running to her tank.
And, oh yes, it's good for cleaning, too! Clean slowly to interest and not scare your octo.