We have been on a lot of cruises (and love Princess, by the way), although my "baby" was 10 the first cruise she went on. The best advice on the car seat is to call Princess directly -- their customer service has always been very helpful for us. You're correct on most of the land tours -- they load everyone on a big bus, and most of the time, the busses do not have seat belts. I don't know if Cares system would work with a bus seat. Depending on the shore excursion,it's also possible that one bus takes you to the the land tour, drops you off and a different bus picks you up (our Swim with the Dolphins in the Bahamas was like that). Sometimes rather than busses, the transportation is vans, and most of those will have seat belts. It may not be worth the hassle to take a car seat with. Princess should be able to help you with the shore excursions and what kind of transportation is used. I would book the shore excursions early, on-line; many of the best ones sell out soon. You can always change your mind up until about the day before. Enjoy the cruise, they are a great way to travel!!!!
D. G, from Las Vegas