Definately do not ride I-5. I've ridden 99 from Portland to Salem and found it to be pretty good, although I've taken smaller routes and found them to be better.
Heading west, 99 isn't too bad going through Portland--- it's often high traffic, but you have a bicycle lane all the way. From there you can hit 22 which will take you most of the way to the coast... I've never ridden it, but don't think it would be too bad.
Going west, I've done Portland-Astoria via federal highway 30. This follows the Columbia river, ending in Astoria at the Northwestern tip of Oregon. On the section I did, there was a quite comfortable shoulder most of the way there. Towards Astoria it gets quite hilly, but most of the way is relatively flat. There's also the most randomly placed Korean restauraunt I've ever seen.
I've ridden some of the smaller roads that you could get to via 99-W that connect the coast to the Wine Country area of Yamhill, Carlton, etc. Very pretty and low traffic, from what I've found.
If you have any questions on specific routes/highways, put them forth and I'll see if I can answer them. Another resource you can visit is
www.bta4bikes.org, they have some maps and if you contact them they can answer your questions.