My two bimacs were not very interested in any toys. They liked interacting with me: tugging on the feeding stick, squirting water at me, pulling on my hand (and biting it if I let them take it home). They also go for the cleaning magnet when it's moving. They like moving things that they can try to kill and eat, or at least wrestle with. I think that chasing live food is by far the favorite hobby of any bimac (probably any octopus). A tank mate that won't nip at the octopus, but can evade capture for at least a few days will provide hours of enjoyment (for the octopus). There is a little red shrimp (Lysmata californica) that lives under the same rocks where I find little bimacs, and evaded capture for months. I think they evolved together, and so are matched in their ability to chase and to evade. Maybe you can try various small tank mates when your bimacs get bigger, but such an experiment would probably be too risky while they are so small and edible.
I also think my adult bimac likes to watch me and the family go about our daily business (the tank is in the living room). I would swear that my first one liked watching TV). When the house was empty for two weeks (except for the octopus feeder every three days) my bimac must have been bored without anything to watch, and kept ripping the tunze nanostream pupmp off of its magnetic mount and pulling the cover off. He did it every three days, but not again after the family came back and there were people to watch every night.