If you're starting in Portland, the Upper Clackamas might be worth a look, or the White Salmon in Washington. Heading south you could check out the Santiam outside of Salem, the McKenzie outside of Eugene, the North Umpqua outside of Roseburg, and there's a handful of day runs on the Rogue in southern Oregon. I'm not familiar with the companies that guide on the Santiam, McKenzie, or N. Umpqua. Down south there's a handful of companies in Ashland. Momentum, Noah's, Indigo etc. All are good. And agree with WWE, the Upper Klamath is a blast if you're looking for a bit more thrilling whitewater.
You mentioned you will be heading down the coast. Not sure if you mean right on the coast, Hwy 101, or just down I-5. Keep in mind, most of these rivers are east of I-5.