Mount Diablo dominates everything here. The mountain rises alone to 1,173 metres above the eastern side of San Francisco Bay, its brown summer slopes dotted with dark oaks, and on a clear winter day, after rain, the view from the top is said to be one of the widest in the world, reaching the Sierra Nevada to the east and the Golden Gate to the west. San Ramon, Danville and the gated enclave of Blackhawk spread along its western foot, a valley of tidy suburbs, office parks and golf courses.
The valley has a few surprises. In Danville, the playwright Eugene O'Neill built Tao House in the 1930s with his Nobel Prize money and wrote Long Day's Journey into Night and The Iceman Cometh there; it is now a National Historic Site, visited by shuttle. At Blackhawk, a museum houses a collection of classic and rare automobiles, a fine detour on a hot afternoon for anyone who cares about engines.
The swimming is a short ride south. The Tri-Valley summer is hot and dry, often well over thirty degrees, and locals cool off at Shadow Cliffs, an old gravel quarry near Pleasanton turned into a lake with a swimming beach, or at Lake Del Valle, in the hills south of Livermore, a long reservoir with sandy beaches, lifeguards in season and oak-covered hills all around. Check whether swimming is open before you go, because the parks close beaches when water quality is poor.
The riding is the main draw. The roads up Mount Diablo, North Gate and South Gate, climb through grassland and chaparral to the summit with tight bends, steep grades and big views, and they are popular with cyclists, so be patient. Morgan Territory Road, a narrow single-lane road over the hills east of the mountain, is a classic for riders who like it slow and twisty. Crow Canyon and Norris Canyon Roads cross the hills to the west with nice curves. Watch for gravel, deer and summer fire closures.
San Ramon has fuel, food and services everywhere, and Danville has a pleasant old downtown with cafés and restaurants. Fill up here, drink plenty of water, and if the day is hot, find that lake.