Corals are typically not a great idea, most require much higher light levels than an octopus. There are a few low light coral species, but you have to be careful about stings as octopus skin is quite sensitive, also octopus like to rearrange their environment, so the free living corals like Mushrooms may get slammed!
Octopus really do best in a species tank, with some clean up crew (although crabs generally end up as dinner ). We keep giant seven armed stars (Astrostole scabra and Kina (NZ sea urchin Evechinus chloroticus) in with ours. Occasionally we may add a greenlip mussel line (from a farm Perna canaliculus, interestingly the octopus doesn't touch this, stars love it tho' ), we also have various macroalga's in the tank. Very occasionally we have spotted wrasse (Pseudolabris celidotus) in the tank, but fish are not usually recommended (either end up as octodinner or annoy the octo). All of these are NZ species, but I'm sure there are similar in your neck of the woods. Oh and we have a very large tank (1200L) for these, they're all pretty big (even the octopus!)
I use a rock with several christmas tree worms, a couple VERY small anemones, and it is truly amazing watching mine hunt chromis every month or so. The hermit crabs, he doesn't seem to want to eat, just throw them around the tank for fun.
Do Not!! I repeat Do Not put a chocolate chip starfish in with them!! I lost an adult blue ring and a juvenile o.vulgaris (in 2 seperate tanks) to these assassins! The blue ring was cornered in a piece of live rock an eaten alive!! My friend witnessed this while I was out of town. I have had success with a coral banded shrimp in my 30gal cube along with a sub adult bimac; a juvenile mimic; a long spined sea urchin and a maroon clown. The octos chase each other at times, but never get into any altercations. I also have a 14gal biocube with a blue ring; a smile spined urchin; a blue damsel; several blue legged hermits (occasionally get eaten!) and several small nasarius snails.