Only Mother Nature knows the exact dates to enjoy the fall foliage colors while hiking or biking, because how quickly leaves change color depends upon temperatures and moisture. In September to mid-November, there's bound to be a place in North America where you can put on your hiking boots or hop on your bike and enjoy the changing colors.
Join the group rides during the annual Shenandoah Fall Foliage Bike Festival, Oct. 18-20, 2013, in Staunton, Virginia. Each day there is a variety of rides offered that are different lengths and for different skill levels.
Peak colors in this regions are usually toward the end of October into early November.
Colorado.com has a good guide with drives where you can see the leaves change in the Rockies and other parts of the state. Almost every Saturday or Sunday, one of the chapters in the Colorado Mountain Club has hikes in the mountains on trails where the aspens are turning to gold.
About.com's Guide to New England, Kim Knox Beckius, has fall foliage covered. To learn where to go and the peak times for fall foliage in the New England states visit How to See New England Fall Foliage at Its Peak. She also has a list of companies that offer Fall Foliage Bike Trips.
In a blog on TravelOregon.com, Mikhael Romain has some excellent sugggestions for hiking or biking in prime fall foliage regions throughout Orego. The routes range from right in Portland and along the Columbia River Gorge to trails on cliff tops overlooking the Pacific Ocean.