This is a wonderful ship. You should have a good time. Shore excursions are not included, but generally are good. What ports do you have other than Venice and Rome? In Venice you can do a lot on your own, although if you want to go to Murano or Burano you may want to take a tour just to make the travel easier. In Rome you need to travel from the port to the city, so a tour may be best. They will send you information ahead of time about trips (including costs) and you can book as soon as you get aboard. Don't delay too long as popular trips may fill up. Trips generally range from $40-100+ per person depending on the trip and the duration.
You will need to pay for tips on board (about $8/day per person) but you now can charge these to your room credit card if you wish. Spa and salon charges are comparable to the nicer salons ashore and are also not included. Get reservations early for any salon services, esp. for formal nights. You will also pay extra for any drinks (alcohol or sodas) at either meals, bars or between meals. There is all the iced tea you can drink for free next to the Neptune pool though. Tips for drinks are included in the charge...don't pay extra. Prices are similar to a bar at home.
Crystal has a no-cash policy. You will need to set up a credit account with a credit card upon embarkation. They will not accept cash for anything on ship, even the shops, except for tips. You can pay off your account with a check or cash at disembarkation, but a credit card is usually easiest. They will give you an account settlement statement at the end, but you can get an account statement anytime just by asking.