Take a walk, or a bike ride, at Crissy Field.

Try the pastries/lunch at Tartine (a few storefronts down from Delfina at 18t and Guerrero - no sign on the shop, but you will see the line). They also have a restaraunt that is very good - Bar Tartine on Valencia btw 16th and 17th. All of this is walking distance from the 16th St. BART/MUNI stop.

After the pastries/lunch, hang out at Dolores Park and enjoy the view.