Puerto Vallarta and Mazatlan will both have vendors...lots of them... just off the ship, but beyond that, everything else is too far to walk. Use their cabs if you don't want to do ship's tour. Once you get into town, lots of shopping and beaches, mostly. But both are pretty spread out. Also, in Mazatlan, the place where the ship docks is a fair distance across a shipyard. They will have free shuttle trams to get you back and forth to the gate to the actual city. Use the tram. Hubby hiked it once and nearly got run over by local "stevedores" driving equipment at maniac speeds!
Answers my question.
We have had nothing but bad experiences (probably bad luck) with excursions in the past but enjoyed wandering around and shopping in the towns.
We like to set our own schedule so we'll stick with cabs.
Peace through superior firepower.