We sailed the Constellation twicw, last year for the Baltic's, and this year for the British Isles and Norwegian Fjords. The tours might appear expensive, but taking everything into consideration, the only way to go. You can't book cars on the ship, but people have booked them before. These are not rental cars, but private escorted tours.
PLEASE NOTE, we did see the ship leave passengers on the pier both last year, and this year. 3 young men last year were brought out by a small Naval vessel, and climbed a rope ladder. This year, the people got to the next port themselves. If you take a ship guided tour, they will wait for you. It you don't, the ship leaves without you if you're late. We've had marvelous tours with the ship, and wouldn't do it any other way. Good luck!