I'm loving San Francisco's Presidio these days: new trails and overlooks, an improved campground, and even art!
The Presidio Habitats Exhibition, which runs through May 15, 2011, is described as "A Site-Based Art Exhibition Celebrating Presidio Nature and Wildlife."
If you go to the Presidio to check out the art, consider making a day of it: go for a hike (or two), have a picnic at the beach, or take part in a guided adventure where you can learn about local butterflies (see the Habitats Exhibition page for butterfly info). And look for Andy Goldsworthy's "Spire," near Inspiration Point.
The Presidio is a great, easy-to-reach destination; enjoy nature in the city.