If you have a few hours to kill on a nice day, I recommend kayaking in San Francisco Bay. It's good exercise, the views are great, and riding 5-foot waves is a blast.

However, I do not recommend underestimating the current, paddling farther out than you can paddle back, letting it get dark, falling in the Bay while you're trying to mount the rental place's jet ski to get a ride home, and becoming utterly cold and miserable.