So here is my supersecret (not anymore) route that avoids all of downtown traffic and almost all of barbur north of terwilliger.
If you're going north take a right on Miles st (goes by the white church and community center). Cross over the freeway and you end up on Custer rd. Follow this for 2 block and go left on Corbett.
Now you take corbett way north (down a super steep hill) all the way to boundary street. Take a right (go east) on boundary and cross 43. Now you go back north on Landing Square Dr. This area is full of parking lots, so depending on the time of day there may or may not be a lot of traffic. Typically when I take this route (at night) these streets are abandonded.
So here where is gets a little off the map because you're going to cut through parking lots and stuff. You can make your way all the way to Avalon Hotel and Spa (it's big) and you take a quick little downhill eastward crossing the train tracks. Now take the road that goes to the north immediately after the tracks. Follow this until you meet up with what is the very beginning of the west side trail (Willamette Greenway I think is the official name). Now you can take the regular route to downtown from that trailhead which then puts you on the southern edge of waterfront park.
Hopefully this is illustrative enough to be able to see it on a map.