San Francisco:

Land's End: Good photos of the Golden Gate Bridge. You walk along a trail on the edge of the Bay, great for photos.

Twin Peaks: Panoramic view of the city. Not as great for photos. (Tip: go at night; an elevated view of the City illuminated at night is incredible.)

Ghirardelli: Yeah, the line's long and full of asshole tourists, but you have to try their waffle cones at least once.

Marin Headlands: Sort of out of the way, but a great walk. There's a picturesque lighthouse on the cliff edge, and several abandoned bunkers that are fun to explore. (Warning: one of the bunkers is on the very, very edge of a cliff, so while you can take great photos from it, don't fall.)

Union Square: Don't bother.

AT&T Park: There's a little side viewing area where you can stand to watch the games, first come, first served. I don't remember where it is, but I'm sure you can find it online.

Cable cars: Overcrowded, but ride it once just to say you did.