Wynola - Julian - Cuyamaca & Return - This 40-mile out-n-back doesn't have many flat sections, and the road is too narrow to be divided, but the asphalt quality is remarkable good for such a lightly-traveled road.
This area was flattened by a fire about five years ago, so the oak-canopy over the road is gone. But the new growth is really beautiful and you'll see a few burnt trees sprouting back to life again.
Be sure to bring at least a 39-25 on this one, because there are a few crazy-steep sections. Also, this is more of a site-seeing ride than a steady workout--the road has lots of up & down, so your heartrate will likely be spiking and diving accordingly. There are markets with water & snacks at Wynola, Julian and the south shore of Lake Cuyamaca.
If you find yourself running out of daylight, you can jet back to Wynola on the 79 & 78, but the traffic will seem terrible after the quiet serenity of Engineers Road.