The Sutro Baths ruin is at the very western edge of the Presidio. They were originally built in 1894 by an eccentric self-made millionaire who had a dream to provide healthy, inexpensive swimming for the masses. The site fell to ruin in the early 1900's but they are still a great place to visit.
If you are into urban exploring, you can crawl around the ruins, check out the graffiti and wander in the tunnel.
And if you are into ocean views (and who isn't), go at dusk for the perfect sunset.
You can visit Sutro baths as part of an afternoon exploring the Presidio and all of the trails along the western edge of San Francisco