If you are looking for daylight, around Christmas, you are looking at around 8 hours and 20 minutes from sunrise to sunset. Essentially 0800-1630. Weather is "changeable". A lot like Melbourne in the winter but my observation from having lived in the GNW is a strong front followed by beautiful...
If the snow holds off, you should head for Mt St. Helens. Probably a fair amount of regrowth now but the reshaping of the geography and the matchstick blowdown of trees miles from tjhe caldera are worth a day of hiking around the mountain. Johnston Ridge is the NPS touristy show and tell and...