There are also discounted fares for seniors (you didn't tell us your age).
If you are going to revisit the area multiple times, getting the ClipperCard could be especially useful. You put a stored value on it, then can use at all (or nearly all) of the Bay Area's transit agencies, including, of course, Muni and BART. There are 22 transit agencies listed on the main page of
www.ClipperCard.com (where you can also learn more about how to obtain and use the card, fares, etc.). Of course, if you put a specific pass on it (like a 3-day Muni pass), that pass is only good on that particular agency. But you can also have money (stored value) on the ClipperCard along with one or more multi-day passes.
In addition to my ClipperCard (I'm a local), I also have a MetroCard (NYC), TAP card (Los Angeles), and now even an Orca card (Seattle); all of these are reloadable.