I am a Platinum member, so you can guess that I cruise with Princess often. I love Princess. Overall, it is great, decent food, entertainment, accomodations, service, and all. The only complaint that I have is the expensive excursions, especially for Alaska and Australia/New Zealand. For Western Carribean, the excursions are quite reasonable. Snorkeling, kayaking, and whitewater rafting are fun and RELATIVELY inexpensive. Of course, the cost is less if you do it on your own. Given the limited time and the headache/rush associated with getting back to the ship on time (and safety), it it worth paying a little extra for Princess excursions.
As others have suggested, check out cruisecritics.com and do a search on the top fun things to do in each of the ports. Don't forget to sign up for the rollcalls via cruise critics board. You can ask questions and have discussions with your future fellow cruise-mates.
I just joined my rollcall for Australia/New Zealand. I have already signed up for two private excursions arranged by fellow cruise passengers. Of course, the cost will be less, and there is freedom to do more sightseeing and fun things.