The last thing that we did on our second day in SF was walk across the Golden Gate Bridge...and then all the way back. When we first started walking towards the bridge I thought we were just going to get a closer look, and then I realized we were going to walk the whole way--about four miles. :)
If I had known how awesome it would be to walk all the way across and back, I totally would have planned to do this at the beginning of our trip! We went at the perfect time--we started at 5ish, right at rush hour, and the sun was setting on our way back across. We were able to walk to the same sightseeing spot as part #2, before we hiked down to the beach, just in time to see the sun set. It was beautiful!