Climbing Contra Costa County's Highpoint
Rising a decidedly non-devilish 3,849 feet above sea level, Mount Diablo is not quite the highest summit in the Bay Area, but it is Contra Costa County's highpoint, and, thanks to its remarkable prominence, Diablo offers not just fine views from San Francisco to the Sierra, but in fact one of the largest viewsheds in the western United States.
Jay Anderson calls it "the only true mountain in the area", referencing Mount Diablo's almost total isolation from the otherwise meandering coastal ranges that surround the Bay Area. That makes Diablo feel much bigger than it actually is—and it makes the mountain a worthy peak-bagging target, elevation notwithstanding.
The crux of this route involves not your feet but your car. First, you'll need to pay the access fee to Mount Diablo State Park to gain entry. Second, after navigating an interminably long and winding road, you'll need sharp eyes to avoid passing the start of the hiking trail (at the Juniper Campground) and inadvertently driving all the way to the summit parking lot.
Yes, Diablo is a developed mountain, and in a particularly unsporting twist, the true summit is covered by a visitor center whose centerpiece is a wholly-enclosed 360-degree view tower. Alas!
On the day I visited the views were impressive but marred by the hazy flat light that we typically see mid-day in California's inland basins. A better (ie contrasty) time to visit would be sunrise or sundown, though that may set you in conflict with the park's operating hours. Winter can see snow atop the mountain—18" fell here in December 2009—so keep those boards handy.
From Juniper Campground it's about a mile and less than a thousand vertical feet to the summit. If you want a climb more worthy of the mountain's namesake, try biking up from the 680 freeway—though be advised the road is narrow, and with its countless blind turns, not at all bike-friendly if there is traffic.