We've been to the Mexican Riviera several times. We typically don't do excursions per se, but just walk into the local town. Here's what we do at those ports:
Cabo San Lucas. We always walk to Solimar Beach via the lobby of the Hotel Solimar, and go south to the end of the beach for a little sunning. The recent-most time a Mexican father and son asked us if we wanted to be shown how to go the Honeymoon Beach (which is an "excursion" for the cruise lines). I always thought that one could get there if you don't mind a little bit of "boulder hopping." We politely declined, but they continued on over the rocks. I later climbed to the top of the "first bit of rocks," which revealed that it would take another bit of boulder hopping to actually get to that beach. At any rate, we usually go back to the Hotel Solimar for a couple of fish tacos and a beer before returning back to the ship. To get to Solimar Beach, just go towards the Pacific Ocean off of the cruise dock (it's only about 1/4 mile). We saw whales off of Cabo, plus I witnessed a stingray leaping and flipping near the ship on our last trip on the Golden Princess.
Mazatlan. On our recent visit, we walked the approximately one mile to the Pacifico/Corona brewery, where we were given a very inclusive tour, followed by a beer at the "top of the tower" lounge. They also gave each of us a small momento brochure with our picture included. Cost ..........zero, plus some good exercise. In a prior visit, we just walked to the cathedral for a self-tour, followed by some local window shopping. The Golden Princess seems pretty liberal in allowing passengers to bring beverages purchased ashore back onto the ship. One man in our brewery tour hauled four quarts of Pacifico back to the ship. We only took one.
Puerto Vallarta Puerta Vallarta is very charming, and an easy bus ride (cost about 80 cents each way) from the main highway just outside the public dock area. Take the "Centro" bus, which goes as far as the old plaza before turning around. On returning look for the "Walmart" bus. There is also a Sam's Club and Walmart across the highway from the dock. The old town center has great walking, shopping, nice beaches and the old cathedral. We have also just walked south along the beach from the dock and relaxed on lounges at the many resort hotels along the beach.