I'm a big fan of rides that you can get to from your doorstep, and this is a fantastic one! You can take advantage of the TriMet line 30 (in blue in my map) to whisk you out of town to the very edge of the Mt Hood National Forest.
Travel Oregon calls it the Abbot Barlow Pioneer, but it's the wilderness riding that stands out to me. It takes advantage of so-called "roads" that form boundaries between designated wilderness areas (you can't ride a bike in the wilderness areas). Luckily these are not paved, or even passable by cars for the most part. Only bikes, dirt bikes, and hikers can complete the Abbot Rd portion of this journey.