Local Transportation: Minivans go along the coast to all spots along Matanchen Bay (the village of Matanchen, the entrance to Las Islitas, Matanchen Beach, Aticama, Los Cocos and Santa Cruz). They leave from the corner of Mercado and Canalizo, a block from the plaza.
Tepic and PV: Buses leave from the bus station on the plaza every hour on the hour for Tepic. Some take the new road, which is shorter, while some still go the old route. There are also combis (minivans) to Tepic on the corner of the plaza. There are 2 buses a day to Puerto Vallarta.
Long distance buses: The long distance buses are great, with air conditioning, movies, bathrooms and comfortable seats. I always bring earplugs, eye shades and a light blanket and a small pillow. Usually you have to take a bus to a large hub and from there you catch a second bus. For example, to get to Zacatecas you would take a bus from San Blas to Tepic (1 1/2 hours), from Tepic to Guadalajara (3 hours) then from Gudalajara to Zacatecas (5 hours). You can get there in one day.
There are 2 taxi companies by the plaza and a 3rd on the way out of town. They charge about $150US one way to Puerto Vallarta, $50US to Tepic.
Rental Cars: There are no car rentals in San Blas or surrounding areas. I have heard they have them in Tepic but even that is iffy. Best place to rent a car is in the airport in Puerto Vallarta.
Flying to other parts of Mexico
Volaris and Aeromar fly out of Tepic. To fly to most places you have to transfer in Mexico City although they have direct flights from Tepic to Tijuana. Unless you are going really far it is probably easier to take the bus.