El Sereno Open Space Preserve is part of the Midpeninsula Regional Open Space District, and consists of steep chaparral-clad ridgelines above Lexington Reservoir. The bulk of this hike is in the open so a trip in the summer is not recommended. All of the trails here are wide fire roads, and the return trip to the trailhead can be a doozy if it is warm.
The hike starts at the small trailhead at the end of Montevina Road a few miles north of Lexington Reservoir and Hwy. 17. The Montevina Ridge Trail heads due north and uphill from the trailhead about 1/4 mile to the Aquinas Trail. Go right/east on Aquinas for 1.7 miles as it travels downhill from Monte Sereno, the high point of the hike at a shade over 2,500'. There are great views of Mount Umunhum to the south and Monterey Bay so the southwest along this stretch. You'll reach a junction with the Loma Vista Trail which heads down to the north to the end of a dirt road, and continue on Aquinas for about a half-mile to a junction with the Serenity Trail. Serenity heads down to a lookout about 1.3 mile away while Aquinas continues almost 2 miles due east to the end of another dirt road.
There isn't any shade at the lookout over Lexington Reservoir that lies at the end of the Serenity Trail, but there are 2 trees about 1/4 mile back up the trail that work. Enjoy what little shade there is because the hike back up can be difficult. The route back up retraces the way you came, and it's relentless uphill for about 1,100' over 3 miles. That doesn't seem like much, but it is when it's 90 degrees. You'll mercifully reach a nice copse of bay trees near the top of Monte Sereno that provide much needed shade before heading back down to the trailhead.
This park is popular with mountain bikers, and there are numerous side-ramps, or whatever they're called, that cyclists use to get in a little extra fun, and to slow down their descent.
Check out the Official Route and Triplog.