My favorite artist these days is Andy Goldsworthy -- I love enjoying art and the great outdoors at the same time, and I can't think of anyone who combines the two with Goldsworthy's imagination, skill, and beauty. This British artist has a new art installation, the Spire, in San Francisco's Presidio.
The Spire is an assemblage of bare cypress tree trunks, rising from the earth to a point like an ancient skyscraper. These cypress were culled from the surrounding Presidio forest -- they were cut down due to disease or death, and Goldsworthy was inspired to, well, build a spire from them.
This installation is a very brief stroll from the Inspiration Point trailhead. From the parking area, walk on the side of Arguello toward the golf course, about 400 feet. Then cross at the crosswalk and continue on a wide nearly flat path a short distance to the spire. Here's a map (pdf) and more info.