A casual stroll around Lake Merritt is a lovely way to spend 3miles in a downtown setting. Plenty of people watching. Watch out for the geese.
A plethora of things to do, so we won't list them all out here... but top of mind (also duh): - Walk the Embarcadero and peruse the shops at the Ferry Building to enjoy views of the bay - Stroll down Grant Ave in China Town and get some dim sum - Pier 39 if you can't get enough of the seals - Picnic at Mission Dolores park for a sunny section of the city with party view and a city view - Walk the Golden Gate Bridge. It's famous. - Alcatraz. It's famous. - The Painted Ladies - Everywhere you look - Golden Gate Park (a mini Central Park because they're both rectangular?) has lovely areas and a wonderful art museum (the De Young), science museum (California Academy of Sciences), and Japanese Tea Garden - And if you want to see the Pacific Ocean... head to Ocean Beach or Sutro Baths!
Plenty of hiking trails near and far. A few favorite areas are Sibley, Strawberry Canyon, Skyline Gate, or Claremont Canyon (steep but worth it).
Drive up to Grizzly Peak and pull over on a turn out Marin Ave. and Fish Ranch Road, and smoke a joint and just marvel in the beauty of the Bay Area.
There's so much to offer, but if you're looking to extend we recommend Napa/Wine Country in the North. Monterey/Carmel down South, and Yosemite and Tahoe farther Northeast and Southeast. Closer day trips could be Stinson Beach, Point Reyes/Tomales Bay, or Half Moon Bay.
It's a grocery store, but it's Bill's favorite place in California. It has 30+ varieties of any sort of fruit or vegetable, so... you get it.