Our first library adventure in SLO took place at Arroyo Grande Library in Arroyo Grande, CA. They have a children's area with LEGOs, computers for kids, puzzles and a little private area in the back. Here are the photos that we took from our library visit below. 
The kids really enjoy the LEGOs, although it seemed too close to the main area where everyone could hear them building. The loud plastic clangs that came naturally from the shuffles of the LEGOs can sound disturbing a bit to maybe some. Nonetheless, the boys loved it! 
They have weekly story times here for the little ones. See the free story time schedule here

The address of the Arroyo Grande Library is: 800 W Branch St, Arroyo Grande, CA 93420. You can come here and then visit the playground just a couple minutes drive away.