Congratuations going to Europe on the Millie. Isn't she beautiful!!! You are going to have such a wonderful experience. I booked my trip only a couple of weeks before we left so I booked all of my tours once I was onboard. I am not sure if you are familar with cruisecritic.com but they have a message board and you will probably meet some people online before you cruise. The terrain you are traveling is hilly to say the least. By booking onboard, I missed out on Pompei but that was all.
My favorites were the Grand Canal, Venice; Florence and Santorini. I hope I have assisted you somewhat. When you go to celebrity cruises websites, they do indicate which tours are best according to the terrain.
Have a wonderful time; I wish I was going again in two weeks.
Oh yea, take cards to give out to the guests you meet and want to keep in touch with. I did not and have been searching high and low for one person I met in particular.