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!)
J