here's a somewhat different route to Champoeg than c-t's:
it's a couple miles longer, and involves a fairly hellacious 2 mile climb up Pete's Mt. - but you get to ride the Canby Ferry. Given that time is a consideration, you're probably better off with c-t's route.
Another completely different option is to hop the MAX train in downtown & take it out to the end of the line in Hillsboro, and go from there. Getting a whole crew of cyclists on around 4pm on a weekday can be a little daunting, it does get crowded. From Hillsboro, the ride is really pretty nice - but the nearest campground i know of is 40+ miles away on the Nestucca road. It would be a real scramble to make it before dark, as the last 10 miles or so are a pretty steady climb into the coast range. The may be some B & B options in the Carlton / Yamhill vicinity,as it is wine country.
anyone know of any public campgrounds in this vicinity? Does Chehalem Mt. State Park have camping?
I'll search a little to see if i can come up with any ideas....