I've been hanging out at the Presidio. It's a former army base turned national park. It offers quite a lot in its 1500 acres: history, ecology, recreation, concerts and lectures, and art exhibits. It's always busy with people from San Francisco, the Bay Area, the United States, and all over the world---LOTS of international visitors.
And it's all free of charge. You don't even pay for it with your tax dollars. The Presidio is the only part of the U. S. Park Service that pays for itself. All the buildings left behind by the army have been rented as homes, restaurants, museums, and workplaces. For instance, the Lucas family of companies is now located there (including Industrial Light and Magic). Actually not all the buildings have been repurposed. The bowling alley is still a bowling alley.
The building in the photo is the visitor center. Formerly it was the brig. It still has bars on the windows. Behind it is a view of San Francisco Bay. What looks like a mountain in the background is Angel Island.
Drop by whenever you're in town. Go for a hike, have lunch, visit a museum, and/or kick back in one of those red lounge chairs on the lawn of the Main Post.