Okay, just musing here, but how about... The Tower!
At the surface, you have a floating platform. Let's call it "the island". To get on the island, you have a couple boat-style ladders. (Bonus points are awarded if you have both normal "box" ladders and single-post "christmas tree" ladders.) In the middle of one side of the island is a "giant stride platform" about 3-6 feet high. You walk up a couple stairs onto a 4x8-or-so platform, complete with handrails, that overhangs the edge a few feet. From there, you can do a more realistic giant stride entry. Along one of the other sides is a slightly-overhung "bench" for doing back-roll entries. Also, include a nice bench for general seating.
Okay, now descending from the island, we have one line from each corner. They lead straight down to the corners of a "floating" platform at 15 feet. This platform has a ring of handrails about half its diameter, with the handrails being something like a 1x1 set about an inch or two above the platform. These would come in quite handy for buoyancy checks, as any floaters could hold on while they adjust their weighting. (Bonus points for having a "skylight" in the surface island with a rope and bag for lowering or retrieving weights and such.)
Heading down, at each additional 15 feet (or whatever), another platform is attached. The difference with these is that each has an open "hatch" (or two? one up, one down) through which you can swim (giving it the slight feel of penetration diving without actually being inside anything). Two or three CESA lines can also be set from outriggers off the 30-foot platform to similar ones up on the island, for CESA ascents only (thereby clearing the corner lines for line-controlled descents).
As for location, the tower would be centrally located around the middle of at least a good group of recreational-depth attractions. Lines would run from various levels of the tower to the attractions. For example, to get to the bus, take the line marked "BUS" from the 30-foot floor. To get to the rubble that looks like a chemical plant explosion (lots of pipes and such), follow the line marked "RotD" from the 80-foot floor, which will lead you to "Rupture of the Deep".
Finally, just to make it even better, part of the floatation of each platform should come from a plastic half-drum attached open-end-down about 4-6 feet above the deck of that platform. Divers (eg. instructors) can pop their head inside and have a short conversation without having to surface, whether for entertainment or to discuss a small problem.
(For super-bonus points, run 12-volts from land to the tower, with each "talk box" lit on the inside with a small marine-grade light. At night, it'll look *really* cool.)
Turning off musing mode for a moment, paying by credit card and having *cheap* unimproved tent camping available are *way* up there on my list. The former is just terribly convenient (even though I know you'd have fees, I'm more willing to go to a site where I don't have to find an ATM and guess how much I'm going to spend on admission, air/nitrox, and so on). As for the latter, not having a cheap place to stay has been the reason I've skipped quite a few sites. I don't *want* to pay to stay at a hotel. I want to pay just enough to be able to legally sleep beside my car overnight.
Oh, and I don't care if you require the SDI card, a checkout, or a blood donation, but it *must* allow solo diving. That is non-negotiable. (I don't mind diving with buddies. In fact, I rather enjoy it. Unfortunately, I've had enough dives washed out by buddies who cancelled at the last minute, and I've had enough dives on which I felt my safety was compromised by being required to tag along on unsafe profiles or risk being thrown out. I don't mind being asked to buddy up if there's someone around, but I simply cannot abide being again put in the position where I have to decide between being safe and being "legal".)
I love listening to Pink Floyd and Night Diving 
