I just returned from one of my favorite late spring hikes - the Mary Bowerman Trail that circles the summit of Mount Diablo. It is an easy, relatively level, ~1 mile loop with fantastic views in all directions and the potential for unusual wildflower and wildlife encounters. The trail was formerly called the "Fire Interpretive Trail", but has been renamed in honor of Dr. Mary Bowerman, one of the founders of Save Mount Diablo. Among other things, Mary was involved in the design of the trail - in particular lobbying successfully to have the overlook deck built as an alternative to the much more damaging plan to blast the trail through the rock outcrops.
This is one of the best places I know to see relatively uncommon Bay Area reptiles like Sagebrush Lizard, Western Whiptail, and even the threatened Alameda Whipsnake (please treat these rare animals with respect). It is also a great place for botanizing, particularly in the late spring when wildflowers have disappeared from the lower slopes of the mountain. One of my favorite spectacles each spring is the display of Bitter Root covering the chert slopes on the northeast corner of the trail.