We did the Grand Cayman Island Tour during our July 6th cruise. It truly is a tourist trap!
With that said, it could still be right for you. If you get excited at the prospect of holding a sea turtle - go for it. Want to mail a postcard from Hell to your best friend, you've got to take this excursion.
The first stop is a local shop that sells rum cakes (no, not Tortuga, but some other brand). I don't see where we should be stopping at a store as part of the Island tour. If I wanted to shop (which we did after the excursion), there are plenty of shops at the pier.
Hell is a series of shops that sell postcards and t-shirts with every variation on the theme of hell. They even had a guy dressed up as a devil who managed to say "hell" in every sentence. Th actual desolate area that was the source for the name is really quite small. But, they do have a post office named Hell.
I'd suggest you save your money and try another excursion.