I mentioned above what I thought were the best activities for each port; but of course you can do any of them in each port. Not knowing your interests, it is difficult to say "do this" and "don't do that."
I've never much enjoyed the "tour" type excursions in Puerto Vallarta. The "visits" to see pottery being made and glass blowing and sandal factories and tequila factories are little more than "buying opportunities", which is fine if you actually want the products; but a stroll down the Malecon will give you access to all the shopping you could possibly want. I think the best thing to do in Puerto Vallarta, other than just walking around on your own, is a day at one of the all-inclusive resorts. But then the only thing they have that you don't have on the ship is sand! Also, and I have never done it, but people rave about the zip line.
In Mazatlan we loved the guided walking tour of Old Mazatlan, with the Cathedral and the marketplace. Then lunch and shopping at the Golden Zone; or lunch and beachcombing on Stone Island.
In Cabo, the beaches and the water sports are your best bet; although some are happy just to visit Cabo Wabo, Sammy Hagar's famous Cantina.