The Lower Keys (Big Pine to Key West) offers a wide range of fishing opportunits especially for the DIY fisherman with a kayak or canoe.
There are some decent places to walk and wade for fish including the west end of the Seven Mile Bridge, rock bar on the west end of Missouri key and at Bahia Honda Sate Park (lots of swimmers, fish early). You can wade the beach on Big Pine Key off of Long Beach Road.
If you have a kayak or canoe there are lots of places to explore, the Lower Keys are full channels and small flats yu could spend a long time exploring by kayak here. Make sure you have a map or chart with you as it can get very confusing in the back country.
As with any DIY trips make sure you have plenty of Google Earth maps with you to investigate on your own.