Stone Island is a lot of fun. You do not need a tour. As you leave the cruise ship terminal, turn left and walk about 1/2 mile. Look for the little "embarkadero" sign. They will take you there for $2 p.p. round trip on a little panga. The island is very close to where the cruise ship docks. You'll have a great beachy day at a very low cost. There are some very nice beach restaurants there too!