Several companies run day-trips from Dubrovnik to a selection of the Elaphite Islands just off the coast. This tour took us to Kolocep, Lopud, and Sipan. The cost was roughly $40 and included a lunch cooked on the boat, complete with wine and/or soda. The food was actually quite good, although you can go on the trip for around $10-$15 less if you opt out of lunch and bring your own (we didn't know about this until it was too late). So if you want to save money it's a good option. You have about 45 minutes on the first two islands and an hour and a half on the last. You can also take the ferry just to Sipan for the day which is less expensive. These islands are beautiful and I highly recommend visiting one or several of them.