Insta, my fave is the Nestucca River Rd. --- take the MAX to the end of the line in Hillsboro, then follow these maps/directions:
about 45 miles to the Dovre BLM campground. A beautiful and quiet ride (other than the first few miles out of Hillsboro). A pretty good climb for about 6 miles or so up over the crest, then down the Nestucca to the Dovre campground.
I also like the Gorge ride, out to Ainsworth State Park, that was posted. Not as quiet/low traffic as the Nestucca, but great scenery & showers at Ainsworth SP. Not sure how far that one is...probably less than 30 miles from the end of the MAX in Gresham. Here's the maps for that one:
don't forget to report back on your ride... enjoy!