Take the train to Ãle Rousse or Bastia.
Take the 4hr round trip to Scandola by boat, which can be booked on the Marina you can't miss it. Scandola is an island, now a nature reserve, that boasts, among other things, coral, birds '450 seaweeds', etc. Best to book the afternoon trip to avoid being hit by Calvi's infamous winds, which tend to make the trip less than pleasant.
Go diving around Calvi's corals can be booked on the Marina, look for the boat with all the diving suits hanging off it, toward the eastern side.
Hire Kayaks or Dinghys or Jet Skis on the beach.
Hire Catamarans from the Marina.
