I would agree with May. I've done that drive ALOT, but never in November or the height of winter.
Portland is wonderful!!! So many fun things to do and a TON easier to get around in compared to Seattle: OMSI (Oregon Museum of Science and Industry)
http://www.omsi.edu/, Oregon Zoo
http://www.oregonzoo.org/, Rose Garden, Japanese Garden (this is the best one I've seen on the west coast-it's huge!)
http://japanesegarden.com/, Childrens Musuem, lots of great restaurants, rent bikes and bike along the Willamette and across on of the bridges, ride the sky tram that links a parking lot and the big Health Sciences University, hike in Forest Park, visit the PIttock Mansion....
The Oregon coast is a wonderful drive, but the weather can be dicey anytime of year. I've done parts of that trip when all we saw was fog and got poured on in the Redwoods in May (I mean drenched). With either May or November I would definately make sure you have a flexible itinerary in case the weather doesn't coorperate.
Another trip through Oregon would be: from I-5 out to the Columbia Gorge (beautiful!) and then south down through Bend Oregon. Outside of Bend is the High Desert Museum (a gem) and the Newberry Volcanic Monument. LOTS of fantastic things about volcanoes, geology, rocks. You can drive up to the top of a cinder cone, walk on an obsidian flow, hike a milke long lava tube. Sun River has wonderful biking trails along the river. This part of Oregon is on the Eastern Side of the state (on the East side of the cascade mountains - so the weather tends to be drier).
From there we then went down to Crater Lake (we went in Summer though, Crater Lake may not be accessible in either May or Oct due to snow at that elevation)
Sacramento has the California State Railway Museum (seriously one of the best museums I've been too. One end of the transcontinental railroad was here )
http://www.csrmf.org/ . Olde Town, an Art Musuem
http://www.crockerartmuseum.org/, the State Capitol.