If you're interested in Champoeg, here's how I'd go:
Avoids all of the 99W damage. Nice route out of town (Oaks Bottom path and River Road). Low traffic, scenic roads south of Oregon City. Potential snack stops at Oregon City and Canby. Bike path from Butteville into the park. Only 36 miles to Champoeg and 43 to Newberg. And avoids all the awful suburban sprawl on the west side (Beavertron, Hillsboro, Tigard, Tualatin, etc.)
I agree with the poster above that the area to the south of Hillsboro-Forest Grove is very nice, but the stretch from downtown Portland out to Hillsboro sucks any way you slice it. This route has good riding the whole way. Have a great tour, and enjoy your stay in Oregon!