Blake,
Yes, adding mushrooms and whatnot in the form of clean-up crew critters will allow you to increase the food in the tank and keep an active (vs stable) cycle .
There are a few really nice serpent stars you can add that can be entertaining pets in their own right. My favorite is one commonly called a
red brittle star, Ophiocoma wendti. These guys are quite hardy and often help you locate your octo as they seem to move in. We have them in all our octo tanks and they are all named, Pesky
as they will often try to take the octos food, sometimes right from your hand. They are easy to hand feed but oddly they seem to disappear if their is no octo in the tank.
A single pencil urchin is a good choice for rock cleaning BUT they will eat the coraline (purple) algae. They can be put in fairly early after you have good water parameters. I have not experiemented with pincushion urchins but have seen them be successful with octos. Avoid long spined and rock urchins though as there is too much risk for damaging the mantle. In itself a small cut or poke is not serious but the high likelyhood of infection can be lethal.
Leather corals are octo safe as are gorgonians but wait until your tank is well cycled. If you add gorgonians, take care to place them well in the waterflow but NOT in an obvious octopus hunting path. Octopuses do not go around obstructions.
Snails and hermit may become food but can be added early on for clean-up. Once your octo is eating well, it will likely leave these alone unless very hungry (or about to brood which amounts to the same thing). This is not ALWAYS the case but other than with O. vulgaris (I think they will eat any time and anything that moves
), is common.
You can add small crabs (like mithrax or porcelain) but they will become snack food/dinner quickly, regardless of other feeding.
If you happen to head south, look at the bait shops on your way home (bring a cooler with a battery operated air pump). Live shrimp are interesting to have in the tank and will either be ignored by a small animal or become food at some point (but not necessarily right away if you are feeding the octo by hand). If some don't make it home (or in the aquarium before there is something to eat it), you can freeze them for feeding later (be sure they don't smell).
You can also add a few grocery store clams. I like to let them sit in a bucket of tank water for 24 hours before putting them in the tank. These too may become food but they will help clean (a little, not a lot) the substrate if left alone, otherwise they are a good food for variety.