If you're going to the zoo anyway, you might as well spend at least a little time at Balboa Park, located on Park Blvd. near the zoo.
It's a large "urban culture park" with open space, gardens, walking paths, museums, places to eat, etc. It also contains the Spreckels Organ Pavilion, with free concerts every Sunday at 2 PM, as well as other events.
Visiting Balboa Park is, in my experience, more relaxing than visiting the zoo.
https://www.balboapark.org/