One of the places that I'll miss most while we're away this Summer are the Lyon Street steps. You'll find the bottom of the steps at Lyon and Green street. I live close by, and they serve as my and Rich's free gym membership. We run up the three sections of steps (about 275 steps total) and get a quick but effective workout.
The best part is the reward of a view from the top. You'll see the bay covered in sailboats on sunny days, the Palace of Fine Arts, and the Presidio. On weekends you'll also find people playing music, doing crunches, jumping jacks, and the works at the top. It's incredibly motivating, and I always feel appreciative of being in a city where everyone is collectively staying healthy, and thoroughly enjoying it.
I'm completely out of breath when I get to the top, but never regret doing it.