Although Guadeloupe technically includes St. Barts and the French half of St. Martin, most references to Guadeloupe are limited to Grande Terre, Basse Terre and the offshore islands of la Desirade, Marie Galante, les Saintes and Isle de la Petite Terre. Both ferries and large catamarans make day trips to the different offshore islands from the marina near Gosier.
Marie Galante, and les Saintes are populated islands with charming towns, some scenic ruins and small cafes and bars. Nilces Bar on Iles des Saintes was recently selected by Conde Nast Traveler as one of the thirty best beach bars in the Caribbean. It is a small dockside restaurant with live music in the evenings. As near as I can tell, no one ever visits la Desirade.
The Isle de la Petite Terre is two uninhabited strips of islands with lovely beaches and good snorkeling. For a fee, you can arrange for a boat to take you to the Isle from the the St. Francois marina and let you spend the day playing Robinson Crusoe. Be certain to take a cooler, as there doesn't appear to be a water source or any type of facilities. Off season, I'd negotiate to have the boat wait for you, rather than leave and return, as you may be the only people on the island.
The Isle de la Petite Terre