This is perhaps the best mountain biking area in the region. It has several purpose built trails with varying degrees of difficulty. The Springboard Trail is excellent for beginners.
Information and routes
Team Dirt - Alsea Falls
Team Dirt is the local mountain biking club, and their site has excellent information about the trails.
Bureau of Land Management (BLM) - Alsea Falls Mountain Biking
The land is managed by BLM.
The Mountain Biking Project - Alsea Falls Complete Loop
Springboard Trail (RWGPS) [5 miles; +555']: Good for beginners
Highballer Trail (RWGPS) [7.4 miles; +1040']: Intermediate skill level