I have four recommendations.
>> 1. Choose "traditional dining" if you can, then don't skip dinner in the main dining room.
>> 2. Don't miss the production shows, especially if you have not seen them on prior cruises.
>> 3. Don't miss the special events featured on some evenings, such as the deck (probably "Mexican Fiesta") party (be sure to look at the sculptures on the buffet lines, even if you are not hungry enough to eat anythign from the buffet!) and the Champagne Waterfall.
>> 4. Other than that, join in the activities that are of interest to you. Your interests might not be the same as anybody else's.
As far as shore excursions are concerned, on past cruises I have found the following to be worthwhile.
>> Cabo San Lucas -- any of the boat excursions out to or past land's end (some also include snorkeling or swimming at a beach resort)
>> Mazatlan -- the city highlights tour and the "Shopping, Beach, Lunch, and Show" tour
>> Puerto Vallarta -- the city highlights tour and the "Tequilla Factories and Beach Fiesta" tour
>> Acapulco -- the city highlights tour
Next month, I'm planning to try the new tour to the "Rio Caracol" beach resort (Tour F) in Cabo San Lucas, the "Sail & Snorkel" tour in Zihuatanejo, and the "Acapulco through the Ages" tour in Acapulco for the first time.