Mission Dolores Park

When the weather is nice, you can barely see the grass due to the throngs of people lounging around. Luckily, the park was recently renovated and is big enough that you're bound to find a spot you like. The southwest corner is good for a panoramic view of the city skyline (and usually far enough away from the folks enjoying their herbal refreshments on the northern slope). There's also a nice kid's playground, although it's geared more towards older kids.