St. George's is a very nice port for the cruise ships. Spices are the main export from Grenada; so stock up on fresh spices (especially cinnamon) while you are there. Customs havent said anything about taking spices home. Shopping at St. Georges for inexpensive items are good.
While there visit a few nice old British forts.
Visit Fort St. Georges, a short but very hard steep uphill walk from the spice market and port. Last time I took a short taxi ride up to fort and an easy downhill walk down to port.
There is a very nice large 1884 Catholic Church you can easily see from your ship. Its a steep difficult walk up and be careful of wet roads and traffic on narrow streets with no sidewalk.