Sun Princess is a great ship! We have sailed on her and her sister Sea Princess. I love the size! Not too enormous but large enough to have everything you could possibly want.
The Mexican Riviera is great fun! I have done it twice. You will find that the atmosphere is quite different from a Caribbean cruise. Seems a little more informal somehow.
I highly recommend taking a tour that gets you out of Mazatlan and up into the Sierra Madres. We enjoyed our tour to the small villages in this area better than any other tour we did. I really felt we experienced the culture and people of Mexico on this tour.
In Acapulco, hire a cab for the day and have the driver take you where you want to go at your own pace. Our driver even took us back to the ship at lunch time and waited for us while we ate our lunch and had a bit of a rest! He was great.
We have booked the Sun Dec 6 cruise. WE have done the cruise before on the Royal Princess. Its a good trip - saw whales in Cabo, Cliff divers in Acalpuco. Lots of fun, but any cruise on Princess is lotsa fun.