To tack onto what others have said already Key Largo/Islamorada is the spot for diving conus, far and away. If you haven't dove in the ocean but are comfortable on SCUBA before I'd recommend the Benford as a first dive (a broken up wreck in 40' of water) which seems to aways have great sea life, like turtles, nurse sharks, eels, and colorful fish. French Reef is my preferred reef there, unfortunately the reefs have been really suffering the last 15 years, and there isn't near as much healthy coral and marine life as 15 years ago, but still plenty to see if you haven't dove a reef before.
Once you've done a few days on shallower stuff and are comfortable in the ocean and with diving off a boat my favorite reef is a little deeper, Conch reef is 50 - 95 ft deep and I think it's still healthier than the shallower reefs. Absolutely do not miss the wrecks (really artificial reefs), the Duane, the Bibb, the Speigel Grove, and the Eagle. (Pick one or two) I always see larger marine life like Goliath Grouper, large barracuda, and sharks on these. These wrecks are deeper, they sit in 130ish feet of water, but their strictures vary in depth. I highly recommend nitrox on these dives to increase bottom times, and find a larger tank (either an over pumped steel lp 105, or a steel 120), you will hoover down an aluminum 80 at 100 feet long before you're ready to come up. Also wear gloves on wrecks so you can pull and glide and hold on during heavy current (which are common on these.
One final recommendation, you absolutely must do a night dive (or many) while you're down there, the ocean absolutely comes to life at night. Also the colors of the coral really pop in vibrant reds and oranges at night. Just be sure to have a good primary and alternate dive light. Given the choice I would almost exclusively night dive. One idea is to do a night dive of a site you've already done once or twice in daylight at night as navigation gets a little tricky. My favorite night dive is the Duane, followed by the Benford.