The city of San Francisco is a real gem. If the weather is nice, it's a great place to spend time outdoor - walking, taking a boat ride or just sitting on a park bench.
Fisherman's Wharf is such a great place to have a little seafood snack and just people watch. Lombard Street is so beautiful in the summertime with all the flowers.Going down isn't bad, but the climb back up isn't easy.
If you want to combine exercise with sightseeing, there's plenty of steep hills to tackle.