There are some very good short snorkeling excursions in Key West but for the most part we tend to wander on our own. On one trip we did the Hemmingway thing, that was fun and enjoyable for all ages but as long as you can walk it is far cheaper to do it on your own instead of paying for an excursion. The beach that is close to the port is not sandy, it has a lot of rocks and washed up coral. Once you are in the water though it is really nice. We enjoyed the beach even though it wasn't the typical beach that you come to expect when cruising. The trolley tours are interesting but for us they are a one time wonder kind of thing, good the first time, but not a repeat performance worthy.
In our opinion, the shopping in Key West is really not anything to right home about, you're not in the tax free haven of the Caribbean and dickering for the prices for the most part is taboo. Buy a few trinkets but save the shopping for one of the other ports.
Hope this helps.