T!m gave excellent answers.
Sunday mornings after 8, you'll have to compete with church goers over parking (most of the church folks park right outside of the island's entry to the left, btw). If you take a regular walk, I'd say it would take about 40 minutes or less. If you were doing what we did the first time and kind of look over all the decorations/flowers/ boats etc... and did both islands it wound up to be closer to an hour. It is all on level ground and many of the homes are doing things up for the fourth of july right now, so its extra festive.
The ferry has been in service for 100 years. That is fun to take over to the peninsula where you can find a ferris wheel, a maritime museum, "fun zone" and other small scale attractions.
I prefer the "Balboa Bar" ice cream bar that is dipped in chocolate and rolled in "butter brickle" (toffee).