Holy Toledo, this is lovely: We normally think of the San Francisco Public Utilities Commission as dealing with wastewater and underground transformers and other such nuts and bolts; but as it turns out, they also let you sign up for guided nature hikes along Bay Area reservoirs. It's just a bit of a schlep to get there -- 13 miles south of San Francisco, and about 6 miles from the closest BART stop. But once you're there, whoa, it looks SO NICE.
"A REFUGE FOR NATURE IN OUR OWN BACK YARD" screams the Commission's website, adding that they provide sanctuary for "165 species of birds, 800 species of plants and trees, 50 species of mammals, and 30 species of reptiles and amphibians." You can't just waltz in and start hiking; you have to make reservation first. Check out their list of upcoming events here, and sign up for one of them here. Most are hikes, but others are tailored specifically for bikes, and still others for horseback.
Like we said: SO NICE.