My wife and I did the snorkel/sandbar tour and when we arrived back at the pier, simply took a taxi to 7 mile beach. I do believe that we would have had enough time to shop, if we so desired. I'm not sure about which tour company to take. We simply booked through the cruise line. Grand Cayman is beautiful and the boat ride out to the sandbar takes place over some of the clearest water in the world.
Have a great time!!!