J R.
Yelp
Nice little museum, which, unbeknownst to many, is open to the public (unlike much the the Presidio/Defense Language Institute)--and FREE. The displays are nicely organized (chronologically, for the most part), informative, well-labelled, fairly current, and cover the history & some of the prehistory of not only the Presidio but Monterey as a whole. There was a very informative (perhaps too informative) docent on hand to answer questions. (Note: since this is Yelp, I should mention that if you encounter that same docent, you might want to take his restaurant recommendations with a grain of salt.) In addition to the museum itself, there's a pretty big chunk of land you can explore there without having a military ID card, including some nice park/picnicking areas, the Sloat Monument higher up on the hill (Sloat was the commodore who took control of Monterey, then California's capital, from Mexico, in 1846), a small monument commemorating "pirate" Captain Hippolyte Bouchard's 1818 burning of Monterey, a statue of Father Junipero Serra, and other historic artifacts & markers.
For a nice image gallery of the Presidio's history, see http://www.printroom.com/ViewGallery.asp?userid=caviews&gallery_id=963740
Down side: some of the grounds & monuments are in a bit of disrepair.