The Canyon Trail takes you from Smith Rock State Park parking area down to the bridge at the center of the Park. This is a heavily used hiking trail, so ride very slow and yield to hikers. Luckily it’s basically a road so there is plenty of room for lots of people.
There is a steep trail that most hikers take a short ways down called The Chute, but the trail is off limits to bikes. Instead go straight down the double track.