I have been on cruises with three other cruise lines, and this was my first cruise with Celebrity. I was not disappointed by the ship and the cruise, but I was not really impressed either. We had an Oceanview cabin. I liked the large window. I did not like that there was a connecting door to the next cabin, and we could hear every little noise our neighbor made. We were pretty underwhelmed by the buffet. It was the least exciting of any ship I've been on. The lunch and dinner buffet mostly served the same food, and it was the same small selection every single day and night. They really push their specialty restaurants that cost more money. Every time you turn around on the ship, someone is trying to sell you a dinner reservation. The food in their main dining room is absolutely excellent. I was never let down by my meal. I would not recommend their shore excursions. They were a huge rip off. Every one was super rushed, packed with people, and way too expensive. The only shore excursions I was happy with on our trip were ones that we booked with outside parties. I would suggest doing your research ahead of time and booking your own tours through local companies.
Do not bother with an excursion; the main sites are very easy to do on your own. Walk, take a tram, or take a taxi to the old city. Hagia Sophia, Blue Mosque, Basilica Cistern, and Topkapi Palace are in the same area and the Grand Bazaar is a short walk away. We were there on a weekday in the off-season so we didn't have any problems with lines or crowds. We were able to do all but Topkapi Palace in a half day. Our time in Istanbul was cut short for security reasons, so we had to rush a bit. I suggest two days in Istanbul would be sufficient to see the sites.
We had booked a snorkeling excursion but the weather didn't cooperate and it was canceled. So we walked around the Venetian port in Chania, ate at restaurant, and went shopping. Not terribly exciting, but it is a cute town and a nice place to relax.
We took the train to Olympia from Katalon. It was 10 euros a person round trip. From the port, go right and keep walking to the other side of the town until you see it near the water. It was a nice and easy way to travel, but unfortunately they only had one departure and one return, and they only gave us a couple of hours in Olympia. I really enjoyed the ruins and museum and would have liked another hour or so. There is plenty of signage at the site so I didn't feel like I was missing out without a guide.
Dubrovnik exceeded my already high expectations. It is beautiful both from a natural and history perspective. We took the ship's shuttle ($12 per person) to the old city. We started by walking the wall and then ventured into the city to eat lunch and poke around. I wish we had several days there. I don't see any need to book a tour since it is so easy to walk around.