Mount Diablo State Park is off I-680 Diablo Rd. exit, then 3 mi. e. to Mount Diablo Scenic Blvd. and the South Gate entrance station. At 3,849 feet, Mount Diablo isn't all that impressive from an elevation standpoint. But because this mountain is surrounded by broad, flat valleys, the view from its summit is remarkable—under the best conditions you can see almost 200 miles away, a 40,000-square-mile region that includes the Sacramento Valley, parts of the Sierra Nevada mountain range and up to 35 California counties.
The views from Diablo's upper reaches are best during winter and early spring, especially after a storm has cleared the air. The sky is frequently hazy during the hot and dry summer months.
The 8-mile drive on South Gate Road to the summit is rewarding. As you ascend the mountain, pull off the twisty, two-lane road for panoramic views of rolling hills, some cloaked in a blanket of tawny grasses and others dotted with oak trees. Spring (especially April and May) is the best time to see yellow cow parsnips, pink shooting stars and other wildflowers that fill the meadows with color. Resident wildlife includes deer, rabbits and coyotes. The road has many sharp turns and also is used by cyclists; obey the speed limit and exercise caution.
There are hiking trails throughout the park. Only the fit should attempt the 6.5-mile Grand Loop Trail. This loose gravel trail encircles Diablo's summit, has a 1,700-foot elevation gain and includes some hair-raisingly narrow portions that wind along cliff faces. Rock City, accessed via South Gate Road (watch for signs to the parking area), has narrow, crisscrossing walking paths that lead past unusual sandstone formations—many of them pocked with small caves—and meander through forested meadows.
In addition to hiking, outdoor activities include nature photography, horseback riding and camping. There are historical exhibits at the Mitchell Canyon Interpretive Center in nearby Clayton.
Dogs are allowed only in developed areas (picnic sites, campgrounds, paved roads) and must be on a leash at all times.