From the North Peak summit it's about .8 mile down to Prospector's Gap, where you have three choices, depending on how strong you feel:
a) If you're gassed, take Prospector's Gap Road, which is mostly downhill, for 1.3 mile to Meridian Ridge Road, then bear left and go another .3 mile to Murchio Gap, the next major trail junction on this route.
b) If you've got a bit more climb in your legs, the Bald Ridge Trail offers a much prettier 1.3-mile route to Murchio Gap.
c) If your legs feel fine, it's 1.5 miles (one way) to the main Diablo summit, which has running water and flush toilets. Mind you it's 800 feet more elevation gain.
From Murchio Gap, it's all downhill for 2.6 miles along the Back Creek Trail, which tracks due north from the gap (a left turn if you're facing north). Back Creek Trail is another Mount Diablo gem, steep and rocky in parts, but generally a pleasure to hike.
Back Creek Trail takes you back where you started; expect to be exhausted but at least a bit elated at finishing one of the region's essential hikes.