Go to Thee Parkside in the Dogpatch/Potrero Hill area for brunch, and just hang out there! It's a bar and music venue, and it's great as both. But during the day, it's just a cool place to sit and listen to music and have brunch on the back patio.
Then I'd just explore the Dogpatch neighborhood. I like the antique stores, but there are a lot of stores there, so you can shop around before or after brunch.
It may look like a dark and grungy punk venue (and it is) but it's also a pleasant place to grab Sunday brunch outdoors.
Thee Parkside's new coffee bar serves up sunny vibes.