Been a few years since we have been.
Nicest place we stayed was ocean key resort in old key west - great location.
On way down i'd recommend a night or two in or nr key largo. There is john pennycamp state park - we did a glass bottom boat tour. Eating wise we enjoyed a diner called harriets for breakfast and mrs mac's for dinner.
The drive is spectacular - over the 7 mile bridge. There is also a pub on a key nr key west called no name pub - you sign a dollar and pin it on the wall.
Key west itself has a little train you can use to get around if you want (we didn't use it) Very lively at night - lots of bars. There is also the square where they do the sunset celebration and a garden with statues of people from key west.
Where do you fancy on the way down to the keys? We like Miami but again few years since we have been. There is an art deco outdoor pool at coral gabels which would be on my must do list aswell as a south beach.
If you are a shopper there is an outlet mall south of miami enroute to the keys. Always used to get decent bargains there was really quiet.
There used to be a fodors guidebook for miami and keys - not sure if they still do it. Edited to say yes there is a 2013 version on
amazon called fodor's south florida.