There are a lot of great options between and including Rampart Range road and Lost Creek Wilderness. A starting point would be to look at the old Bailey Hundo course.
http://www.bailey100.com/hundo-course.html It did stuff in Buffalo Creek and then hit the back roads through South Platte, Deckers, and Wellington Lake. You could add and subtract as you see fit. You could also include Goose Creek Road, Stony Pass, CO126 (Not gravel) , HWY 67. The gravel roads link above is pretty useful.