Stow Lake used to be one of my favorite places in SF, and then it really got a little seedy around the boathouse area... but now under a new concession, the Boathouse is clean and inviting and has new boats and some new food items. The coffee - believe it or not - is great. We didn't try the food.
We walked around the island and took in the pagoda, waterfall, and the beauty of the wooded areas, and then rented a paddle boat - $19/hour - and slowly paddled our way around the lake, enjoying the ducks and turtles and the sheer peace of the place. It was a weekday, so we were nearly the only ones on the water... if you're going at a busy time there may be a wait for a boat.