Walk down to the Embarcadero and look out at the water and back at the skyline.
Take a free walking tour through San Francisco City Guides. I actually took two, since I had a full free day on Friday, and they were both amazing. One was called "City Scapes and Public Places" and we learned all about rooftop gardens and other green areas open to the public.
I also accidentally walked all the way up to Coit Tower. I saw it from a few streets over and thought to myself "Huh, that doesn't look too far away." But it was actually so, so far away and involved so, so many stairs. By the time I realized this I was too far along to quit. My legs are still sore.
But the view from the top is totally worth it.
I went to Chinatown on a quest for red bean paste buns. This was a treat I fell in love with after discovering them at a little bakery in Chicago's Chinatown. I found a place called Golden Gate Bakery on Yelp and hoofed it over to Chinatown one afternoon between sessions. The bakery was small and kind of tucked away and I almost walked right past it. I was definitely the only non-Chinese person in there and one of the tallest at 5'2", but it was one of the best red bean paste cakes I've ever had. Worth it.
I also walked all the way to the top of Lombard Street, and the view was totally worth the burning calves. See how far away Coit tower is? Yeah. I probably walked 10 miles that day, uphill both ways.
Some very nice and well-meaning foreign tourist asked me if I wanted my picture taken at the bottom of Lombard Street. This is what we came up with, and the only proof that I was actually in San Francisco instead of stealing someone else's images off Instagram.
Other things I did include going to Fisherman's Wharf (do not recommend), eating In-n-Out Burger (highly recommend), taking a cable car (kind of recommend), meeting a friend for delicious sushi, shopping in Union Square, drinking lots and lots of local coffee, and walking miles and miles every day while loving the fact that San Francisco is on a grid and I always knew where I was going. Man, I love cities, and San Francisco sure is a fine one.