Balboa park is a nice but of nature and culture in the middle of the city. There is ample parking in the lots and on the streets which makes this an easy non-stressful escape.
Take a walk down the main street and contemplate visiting the various museums - the cannibal exhibit is one not to be missed ;), but many cost money. The arboretum is free and has many nice plants from around the world.
This is a pleasant place to spend a morning.
There is a lot of sun so make sure you dress accordingly and bring that spf.