There are the "usual suspects" which are great, and you have listed them (but you left out Molasses, which is a truly beautiful site and better overall than the Benwood). However, if I were to list three "DON'T MISS" things, they would be, in this order:
1. Carysfort reef, both shallow (15-30) and deeper (60), and even snorkelling around the light which has a large very shallow reef area close by. I think it is the best of the best. You could do your entire morning with one deeper dive, one shallow dive, and snorkelling during your surface interval. You absolutely will not be disappointed. Use an operator located on North Key Largo. I think Elena above could hook you up with Carysfort.
I would rate Molasses as almost equal to Carysfort in terms of beauty. it is the most popular dive for good reason, but sometimes gets crowded on weekends. There is also a 60' or so dive at Molasses. I would rate this right behind Carysfort if you cannot get over to Carysfort.
Then do an afternoon trip to The Elbow/Benwood or French/Benwood (I prefer Elbow) combo, then a night dive anywhere, and you would have an incredible day.
2. Drift diving with Conch Republic Divers on Tavernier Creek. They are the only operator I know that regularly schedules drift dives, on a reef that is about 50 sloping down to 75 and on the edge of the gulf stream. Let me tell you that dive is a real treat. They then give you a second dive on a shallow reef, and they know a great assortment. However, I don't know if they go as far north as Molasses. The shallower reefs south of Pennekamp can be really excellent, but they are not at the same level as Molasses/Carysfort. If you did the dive with Conch, you would have to drive to another operator to do the Pennekamp reefs for your afternoon dives.
3. If you are qualified, the Spiegel Grove. With your # of dives I would recommend getting a dive guide which most shops can supply at modest cost. This is a deeper dive and can have strong currents. Sometimes you drop and drift to the wreck, and then drift away for your ascent. It is an advanced dive in every sense of the word, but not really dangerous if you do not penetrate the wreck, which is so big you cannot even cover the exterior in one dive. If you want a five dive day (including your night dive) with one on the Spiegel, you have watch your dive plan because you might be pushing the limits (big help if you dive nitrox and keep the other dives to the 30-40' reefs). That being said, if it is your first time in Key Largo, the true magic is in the reefs. Stick with the reefs and you can't go wrong.
