The current, rather frothy debate about proposed development at Crissy Field in the Presidio at San Francisco's Golden Gate National Recreation Area, which arguably ranks in significance with New York's Central Park and Washington, DC's National Mall, deserves more national attention.
New York City has Central Park and San Francisco has Golden Gate Park (which is actually bigger. Just saying). With lakes, gardens, fishing, hiking, wildlife, museums, and recreation areas, the park really has something for everyone.
Lunch Break! Since you are in SF it is likely flexible. Maybe you like yo-self a little noon workout... Fancy an indoor stationary bike ride with a group of sweaty dentists? Suggestion alert! Trade that silly class for a hilly pass!