Marathon Key ? Why not try and go more south or west into the lower Keys?
Like the keys resident said your trip has allot of factors and believe it or not there is TONS of stuff to do in the middle of the keys!
I recommend feeding the Tarpon, lots of fun for everyone even if you are from Miami like me. The trick is to try and grab the bottom lip
I try and get some pictures up if they aren't already corrupted on my comp. I spent allot of time camping in the groves ( in winter) and allot of time near big pine with an added two summers at sea base one as camper and one as counselor.
Flats (fly) fishing is my weakness... Permit and Bones to be exact, but as the keys resident said the fishing trip and time of year and location depend on many other factors.
http://www.youtube.com/watch?v=L0ncYCEXYYk