Welcome Cody!
A 75 gallon tank is a great size for anything but a mercatoris (a dwarf species that you would never see in a tank that large). I just compiled a
short list with links to some of our tank buildouts located at the top of this forum and would invite you to start a thread on your own tank adventure to add to the list.
Hard corals or any coral with a detectable sting are advised against. Octos don't pay a lot of attention to things "in their way" and will crawl over most anything, especially in the confined space of an aquarium. Stinging corals can cause skin problems and ultimate difficult to treat infection.
To add interest and a little color to my octo tanks, I use red mushrooms, a large poly that I can never remember the name of
(sometimes called giant sun polyps), gorgonians and leather coral. Keep in mind the octos carelessness when arrangeing anything that is even a little delicate (like gorgonians) and place them well out of an obvious path. The mushrooms seem to withstand being walked on (I even had one senescent animal try very hard to pull one up without success and without damage).
You can also keep interesting serpent stars (avoid the green ones but there are a number of color and pattern choices) and brittle stars. The serpents will stay hidden most of the time but will learn to come out for supper. I keep a red brittle (actually more orange than red) in all my octo tanks. For some reason when there is an octopus in the tank the animals are almost always visible and active. When the tank has lost its primary residence, you can't find them. These guys have a bit of personality and they all get some version of the name Pesky at our house. I tend to keep one brittle/serpent for every 20 gallons. They are especially useful as a clean up crew for anything that gets behind the rocks.
I keep a couple of pencil urchins in the octo tanks to keep down the algae. Several keepers have also kept the pincushions but I have avoided this kind in with the octos ( I do keep them in other tanks). However, I have not seen any reports suggesting they are a problem for the octopuses and many are quite colorful. Avoid long spined or hard sharp spined urchins (like the rock urchins).
Octoproofing your top is another consideration you will need to address before adding the primary resident. This forum has several discussions on top buliding and securing.
You will want to add a skimmer to your set up. Aquarists will debate the merits of a skimmer in a coral tank but there is no debate when it comes to inking and all octo tanks should run one. If you will be using a sump, adding a skimmer is usually pretty straight forward but working around one in the main tank offers some challenge.
We also keep a list of the octopuses species and keepers located at the top of the Octopus care forum. Look for List of our Octopuses 20xx. The listed species and animal name is a link to its journal.
For more reading pleasure,
Cephalopods Octopuses and Cuttlefishes for the Home Aquarium was written by two of our staff members and is an excellent introduction to keeping the animals and their needs.