Maybe ride up Red Bluff Rd? It's not a mountain biking trail but it is gravel and low traffic. It runs along the canyon wall of rock creek. You could even dismount your bike and find a nice trail going down to the creek to hike.
There is a Multi Use Path at Cascade locks. It starts right under the Bridge of the Gods and goes to Eagle Creek I believe.
It's not biking but the Pacific Crest Trail is right there in Cascade Locks. That would be a fun hike.