Honestly Key West is my east liked area of the keys. Worth going to see once, but not my thing. If you like bars and music, it is probably great.
For me the keys are all about Water activities. Snorkling,fishing, kayaking etc. There are snorkeling place right on the beach in Key Largo and Bahia Honda,and probably lots of other places also. We really enjoyed kayaking through the mangroves and around the islands. It was really pretty. The fishing is incredible, you can go offshore, or inshore if you are worried about seasickness.
It is an incredibly beautiful area, just going over the bridges and looking at the water is incredible. I like the first half of the keys the best Largo, Marathon, Islamorado.
I've never been there in the summer, but living in GA, and vacationing in North FL most every summer, it can be very hot and very humid. If you are not used to it, it will such the energy out of you. But June is probably much better than July or August. Also you should have a breeze most days and that will help.
Go and enjoy, just be ready to sweat, ??
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