Two words: urban oasis. Glen Canyon Park, not heavily used by the general public, is one of the cities best kept secrets. The trails are usually pretty isolated and used mostly by dog owners who live in the neighborhood. Grab a camera and some hiking boots for some absolutely amazing views and hardcore nature exploring. Seriously the best "urban" hiking in SF where you'll feel miles away from the city.

If you're into birds, there are great horned owls that are known to hang out in the trees on the trail near O'Shaunessy Blvd. There have also been numerous sightings of red-tailed hawks looming over the canyon.

If you're not so into birds, but rather find fascination in yellow slugs or other lowly creatures, look down as you walk...there are banana slugs that usually hang out under the leaves along the trails. Some of them are quite huge and they seem to be present all year round. There are also all sorts of snakes. They like to sun themselves on the path, so keep an eye out for those, as well as the occasionally spotted coyotes and foxes.

And if you find yourself on the trail towards Diamond Blvd, go all the way to the top. There is a very nice community garden up there.

*tip: if you don't live near-by, you can take BART, or park in the Safeway parking lot. There are stairs that lead you directly into the park.
                                

This park is inside the city of San Francisco which is hard to believe while you are there.  Great trails and dog friendly.