I took the kids to Angel Island today, and we had a great time. It was sunny and windy and not too crowded.
Angel Island is in the San Francisco Bay, and was used for military defense and immigration/quarantine from the 1860s through 1960s. Now it's a California state park, with a 5 mile long ring road and lots of hiking trails, some of which are also open to bicycles. The most interesting attraction is the two immigration museums, but there's lots of old garrison/forts to explore and great views all around.
To arrive, we took Caltrain to San Francisco, then rode a short distance to the Ferry Building. We bought tickets and got in line for the 8:35AM departure. I had worried the ferry might be crowded, but there was plenty of space for bicycles. Once on the island, many other ferry passengers rented bicycles (many, many ebikes). The bike rental concessioner recommended a counter clockwise loop, but looking at the wind map we went clockwise, which worked out great. There were some steep sections that my 8yo needed to walk her bike up, but some strategically dispensed candy (for "extra energy") can give children enormous motivation.
We had planned to come back on the last ferry at 4:30, but the younger ones were tired and we instead got on the 2:35pm. I wouldn't hesitate to go again.
~Michael