We've been there every month of the year for for many,many years. Every month has something different about it. You are right about the temps... figure upper 80's -> low 90's. You'll have a breeze so that tempers the heat.
We prefer the lower keys and Key West. Many good restaurants and places to listen to music. Being a tourist destination, there are lots of things to do.
Marathon also has a good number of things to do and several good restaurants. Islamorada is the self-claimed sport fishing capital of the world. Key Largo has Pennekamp State Park.
Places to stay--- we stay at Blue Water Key pretty much most of the time.
Also near Key West is Boyd's and El Mar. Geiger Key Marina also has several campsites.