The answer, somewhat incredibly, is 3,849-foot Mount Diablo, summit of the East Bay hills. Due to the way the geography of California spreads out from Diablo's base, the summit offers the second most expansive view in the world, after Tanzania's Mt. Kilimanjaro. On a clear day from Diablo's summit you can look out not only over the Bay Area and the Central Valley to the snowcapped crest of the Sierra Nevada, but 300 miles north to Mt. Shasta, up near the Oregon border. I'm told that on a really good day, with binoculars, you can easily see Half Dome in Yosemite Valley. You can drive to the top of Diablo, but the mountain offers a wealth of great hikes and secluded picnic spots.
Kids love the sandstone formations at Rock City.
From the Bay Bridge, take 580 east to Hwy. 680 at Walnut Creek. Follow 680 south seven miles to Diablo Rd. and take Diablo Rd. Though Mount Tam is only 2,571 feet, its location gives it unmatched views of the inner Bay Area. Tam is astonishingly wild given its proximity to the city.