Angel Island is the largest in the San Francisco Bay; with spectacular panoramic views, this California State Park is the perfect spot to take a breather from the city. Originally a hunting ground for the Coast Miwok people, the island has also hosted Spanish explorers, various military garrisons, a cattle ranch, a quarantine station, and a World War II POW camp. Most famously, it was used as a U.S. Immigration Station, processing hundreds of thousands of mainly Chinese immigrants between 1910 and 1940, giving rise to its nickname, "the Ellis Island of the West."
The Immigration and Quarantine Stations both have museums and you can take guided tours of military installations such as Fort McDowell. For a broader overview take the historical tram tour of the island, and for something a little more active there are Segway and electric scooter tours available. If you'd rather go it alone you can rent bikes (or bring your own) and there are various hiking trails to explore. Make sure to relax on one of the sandy beaches, though be warned that strong currents make swimming hazardous. There's a café and a cantina on the island, too; the latter hosts live music each weekend during the summer.
You reach Angel Island via ferry from Pier 41 in San Francisco or from downtown Tiburon. Check out Alcatraz Cruises if you'd like to bundle in a visit to "the Rock."