For an easy nature walk in the Presidio [1], try the Lobos Creek Trail (Lincoln St. at Bowley St., daily dawn–dusk, 0.6 mi, easy). Less than a mile long, this flat boardwalk trail is wheelchair-accessible and shows off the beginning successes of the ecological restoration of the Presidio.
You’ll get to see restored sand dunes and native vegetation, which has attracted butterflies and other insects, which in turn bring the birds to the trail area.
While it’s still in the City, this trail gives walkers a glimpse of what the Presidio [1] might have been like 500 years ago, before the encroachment of the European military.
Links:
[1] http://www.moon.com/destinations/california/san-francisco-and-the-bay-area/sights/marina-and-pacific-heights/the-presidio