Marian R. Bear Memorial Park is located just off the 52 Between I-8 and I-805 in San Diego. It consists of a doubletrack "spine" with several singletrack splits and out&back branches, some of which end at alternate trailheads.
Considering the location, smack in the middle of urban chaos, this park is a surprising and very pleasant find. While there is nothing overly technical, there are a good number of very pleasant singletrack spurs and side trails to be found.
From the trailhead at the east end of the parking area, head east along the main trail, grabbing any marked spurs or singletrack you see. At this time, all side trails are open to bikes as evidenced by the cities' website and trailhead 4X4 post markers. Continue east past Genessee until you reach the 805. This is the turnaround point/park boundary. Beyond the 805 is Miramar which is off limits. Trespassing can result in the loss of your bike, according to literature and reports. Proceed back (grab any spurs you may have missed - they are they best part of this trail system.)
An added perk to this area is the ample shade. This place is just the ticket on a hot summer day. The scenery doesn't hurt, but unfortunately the 52 is always within earshot so there is no escape from the traffic noise without earphones.