I believe that La Guaira has been replaced by Isla Margarita due to civil unrest. I'm a sailing and beach bum type of person so my preference on Grenada is the Party Cruise and Beach Experience and on Isla Margarita, my choice is the Rumba on la Chiva.
In St. Kitts, we took a cab out to Turtle Beach for the day. You can easily get a cab in town and you can save some money by going with others, its about a 45 minute driver, but very scenic. Beach is nice with a restaurant.