We're there for 4.5h, and I'm not bothering to book anything. Sounds like the trolleys are easy to catch day-of, right where you get off the ship. I found this map on another thread, to give you an idea of what's nearby and where the trolley can take you.
http://www.trolleytours.com/key-west/key-west-map.html
I figure if we decide to devote an hour to the shipwreck museum or something else, it's Florida - so we can use our smartphones to buy tix online if something is cheaper that way. But my basic plan is 1) buy trolley tix 2) get off wherever looks interesting at that moment. It's the least planning I've ever put into any trip!
btw, not sure if OP is on the same cruise as me... but mine is at Key West on New Year's day. I suspect that's why we only have the afternoon off the boat.