Michael, here's a route I put in bikely.com for Portland to Champoeg:http://www.bikely.com/maps/bike-path/Portland-to-Champoeg-State-Park
The one problem with this route is that currently the Willamette River bridge from Oregon City to West Linn is closed - there is a bike/pedestrian shuttle, though. There is also a steep but scenic climb up Pete's Mt., after which you're rewarded with a long downhill to the Canby Ferry (one of 3 ferries on the Willamette River).
Anyway, the work-around if you don't want to take on the shuttle or the mountain is:
from Oregon City, find the elevator right in downtown (no fooling, it's save you a few hundred feet of climbing and some distance, plus it's really odd) then follow High St south for a few blocks , LEFT on 5th, turns into Linn St., then at a big intersection with Warner-Milne & Warner-Parrot, find Central Point Rd. and follow it for about 7 miles - then RIGHT (west) on Bremer Rd. into Canby. You can pick up the route above here.
Here's a series of Willamette Valley bike maps you may find helpful: http://www.portlandonline.com/transportation/index.cfm?c=36638&a=316792
Also of note: Willamette Mission State Park , not to far from Champoeg, now has hiker/biker campsites. IMO a much nicer place to camp than Champoeg -