Grand Manan is worth a walk around. The light house is an easy walk from the harbor, assuming the ship comes into the harbor at the north end of the island. I imagine there will be island tours set up by the cruise line.
Bar Harbor is at the northeast corner of Mount Desert Island. The town itself is small, but there are lots of restaurants and tourist oriented shops within 3-5 blocks of the harbor. Expect the town to be VERY crowded in the summer, particularly if one or more of the large cruise ships are in port. Mount Desert Island is also the main location of Acadia National Park. There are lots tours through the park. In addition there is free bus service that serves most of the island (called Island Explorer). The Loop Road route does the coast loop in the national park. You will have to pay for park entry, but the bus itself is free. That loop takes an hour and fifteen minutes. However, you can get off the bus walk around and catch a later bus for the rest of the loop. The Loop Road route runs every 30 minutes. In addition toe the park, there are lots of tourist activities including schooner rides, friendship sloop rides and lobster boat tours. If you like native american art the Abby museum in Bar Harbor (about 4-5 blocks from the harbor) is worth a visit.
Rockland is the largest of the three towns you mentioned. It is more industrial than the others, but there is still a nice tourist area along the harbor and on Main street. There are quite a few restaurants, but not as many as in Bar Harbor. If you like old cars and planes the Owl's Head Transportation Museum is about two miles south of the harbor and worth a visit. If the weather is nice you can walk out the breakwater to the light house. As mentioned above, Camden, which is a quaint tourist trap is about 8-9 miles north along highway 1. Rockport, which is half way between Rockland and Camden has a very pleasant harbor for walking around and is a local center of wooden boat building.
Hope this helps.
