We rented a car and drove around the north shore. Interesting but nothing memorable other than getting lost a couple of times (Hawaiian roads are notorious for their lack of signage). Not recommended since rental cars are very expensive in Hawaii.
Lahaina is an interesting spot with a profusion of art galleries, quaint shops, a nice whaling museum, interesting restaurants, and the opportunity to go whale watching. We arranged our own trip after tendering ashore. However, we discovered that if you don't book prior to arriving, or through the ship, it will be difficult to get a tour before mid-afternoon.
Another fairly good sized city that could be anywhere. We had previously taken the Volcano National Park 8 hour tour (private arrangement) so this time we just did a little sightseeing and shopping on foot.