I'm going to drop what I consider to be the best solution so far (as well as a back up that is equal). You've got an awesome unique tank, you need an equally awesome interior. You've ruled out freshwater and cephalopods (while amazing, are just ridiculous to keep), so let it be brackish. Get yourself some dwarf indian mudskippers. With the room you have you could do a waterfall trickling down the rocks and logs past plants, then a small shore held up by a covered PVC structure. Past that you have a deep tank with some roots dangling from the shore, maybe some mollies or guppies (both can be brackish) or figure 8 puffers (they can be messy though). Watching them dive down and surface will be awesome for you. I had one for a year before he decided to crawl out into the carpet, easily one of the coolest fish I've had and super easy to take care of. With that type of room you could have a whole school (somehow I think herd is the more appropriate word).
Another solution would be to a reef with tons of PVC poles covered in faux rock going across the tank, you could house some awesome inverts and small fish, maybe have a mass of blennies/gobies, it'd be cool to see how they claimed territory in that type of environment.