I took the same itinerary but on the Pride not the Splendor
Puerto Vallarta: beautiful city with a lot of great stuff
Mazatlan: avoid the time share people at the terminal offering free rides. they'll take you to a hotel and try to coax you into some 6 year cruise plan thing
Cabo San Lucas: boring
Been there and done that but not going this time.
Advice: In port, stay on board and enjoy the luxury of an almost private ship. Not really much to see that you need to see in Mexico.
Do it all -- casino-- pool, shows - dances, classes, library, different restaurants.
In Ensenada, do go on the pier to the funky bar and have a drink and people watch. Do not go away from there except back to the ship.
Long Beach is an easy on and easy off location. Enjoy!