We've done the Mexican Riviera on both Princess and the Norwegian Star, as well as on RC's Vision. Ship-wise, we much preferred the Golden Princess for larger cabin for the price, much better food, and better entertainment. Princess also has a very leisurely policy on bringing your own wine or even beer back onto the ship.
We don't normally do "excursions, but choose to just walk into town and see what's what. At Cabo, we just walk about 1/4 to 1/2 mile to the Pacific Ocean side of the peninsula, through the Calimar Hotel, and down to the beach for an hour or two. Very few people and a nice beach, although swimming is risky. We then usually walk back, stopping at the outdoor bar for a beer and fish tacos.
At Puerta Vallarta we walk to the highway from the dock, which is only 1/4 mile or less. We then catch the "centro" bus, which costs about 8 pesos (less than a US dollar), which takes us to the old plaza. Good tourist walking, shopping, and dining if you wish. If you need to buy something there are both a Sam's Club and Walmart almost across the highway from the cruise dock. There are also resort hotels right next to the cruise dock that would probably allow your to sit and enjoy the beach. At least we were not hassled when resting on the lounges on one occasion.
At Mazatlan, we walked the approximately 1 mile to the Pacifico brewery, where they give free tours at various times. I think last time it was at about 11 AM. There were only five of us last time, so we got a very personalized tour. The tour requires quite a bit of walking, plus climbing up several flights of stairs (I'm sure you'll practice on the ship), followed by a cold beer in the lounge at the top of the tower, which offers a great view. One guy that was on our tour walked back with four quarts of cerveza, which was allowed onto the ship with no problem, at least on Princess. There is also a church worth seeing not very far away.
When we were on the Norwegian Star it was an 8-day cruise, so there was also a stop at Ixtapa/Z. We just walked through the local town, which was interesting.