I guess the real rule is, put nothing in the tank that could harm or sting the octopus, could stress him or take his food.
Take the pencil urchin - it gets along fine with the octo. My bimac did not appreciate it when he moved through her den, but she tolerated it. I bought a small urchin as a companion to my little bimac. After about a year, and after Ollie had passed away, he had grown a lot and was doing significant damage to my coralline algae. I started reading about urchins in tanks and learned that one urchin per 200 gallon tank is about right. So, if you have a larger tank to put him in when he grows up, or you don't have coralline algae, a small pencil urchin works quite well for the life of a bimac.
There are lots of things that make the tank look nice - feather dusters, for instance. I read of one octo who snapped off the feather duster and put it by the entrance to his den. Octopuses are not ecologically minded - they use stuff for their own purposes. They think nothing of turning over your rock with mushrooms on it. So, when they get about half grown and start enjoying themselves by rampaging through your tank, nothing is sacred.
Yes, you could try small pieces of coral or feather dusters, but don't be surprised if they don't survive. I have seen some tanks with (non stinging) gorgonian corals and the octo tolerates them. Guess that depends on the size of the tank and whether he has room do his thing in other parts of the tank.
I would avoid sea apples and sea slugs. Some snails will eventually survive. To make your tank look nice, use seashells, maybe some plants, coralline algae. I liked putting in 10 or 15 shore shrimp, since they're not easy for your octo to catch and they add interest. Hermit crabs will survive after your bimac grows up a bit, and they can add interest and color, too.
So no, you don't have to have a bare tank, but your octo is a natural builder and decorator, so you tank may end up looking quite different from the design you started with. (Mine certainly did!)