Running along the bay from AT&T Park up to Fisherman's Wharf, the Embarcadero provides a break from the city's steep inclinations. As such, it's widely enjoyed by locals and visitors on foot, bicycle, skateboard, rollerskate, etc.
Be sure to check out the Vaillancourt Fountain!
The Ferry Building hosts a farmer's market on Tuesday and Saturday mornings. If you're hungry for a burger, Red's Java House has a devout long-time following, excellent views and rock-bottom prices.
Wikipedia: http://en.wikipedia.org/wiki/The_Embarcadero_(San_Francisco)