Check out Katakolotaxi.gr They do tours and the prices are very reasonable. They have great ratings and reviews on TripAdvisor. We are on the same cruise as you, and booked them for a tour. Andrew is very friendly and easy to deal with.
Harry
We are a family of 4 with 2 teenage boys. We have found a full day semi private tour of Athens with PK Travel. If anyone is interested in joining us the tour is 75 euros pp. Just go to PKTRAVELGREECE.COM. Click on Athens full day tours and check out the tour. There's a link with our ship and day of tour that you can click on to join. We chose PK because of the reviews they received on CC and Trip Advisor.
Harry