Northeast of San Diego and La Mesa near Santee, the Sycamore Canyon / Goodan Ranch area has awesome mountain biking. There is a great downhill stretch from the Poway Road trailhead down east to Marthas Grove.
To reach the Poway Road Trailhead from San Diego, head north on US15 to Poway Road (S4). Head east on Poway Road. Turn onto Garden Road. Turn right on Sycamore Canyon Road and follow to end past the Gibbens Ranch Gate (opens at 8am).
Overall, Sycamore Canyon Open Space is the 63rd most popular mountain bike trail of all 132 mountain biking rides in California.