Another vote for touring the Pacific Northwest in the drier months. I live in Seattle, and I wouldn't choose to be here in December, if I had the choice.

December will be cold and rainy (and can often be below freezing, which makes getting around MISERABLE - the city does not handle snow and ice well at all) and many parts of the Olympic National Park will be impassable. We've done the coastal parts of the park in February (in the rain, of course) and there is a bit of hiking that you can do in the Hoh rain forest area and on the northern edge of the park, but most of the park roads and a lot of the park facilities can be closed that time of year, depending on the weather. We were lucky enough to get up to Hurricane Ridge on that trip, but it was just pure luck that the roads were passable.
Pretty much all of the Cascade Mountain (the mountain range east of Seattle) parks are closed in December unless you're heading out to the ski parks.
As for city attractions, they are pretty well all open, but with reduced hours. And again, you will be touring in the rain with 40 degree temps, as an almost guarantee. The nice thing is that you won't encounter the same crowds and lines that you would in the summer during tourist season. The Seattle Center attractions (Experience Music Project, Pacific Science Center, the monorail, the Space Needle) are all pretty good and within a stones throw of each other. There are also Christmas events going on all December at the Seattle Center (skating, Christmas displays and concerts, pagents, etc.) and they are a nice diversion - I live about a 15 minute walk from the Space Needle and I love to take my dogs walking at the Seattle Center on Saturday afternoons at that time of year to get a cup of hot chocolate and soak in all the Christmas cheer.
And lastly, the Oregon coast is BEAUTIFUL, but I wouldn't choose to do it in the winter. You won't get the same views as you would in the sun, and walking around in the rain all day is pretty miserable.
Anyway, I love this part of the country, but I think there are better times to see it. Seattle itself is pretty desolate and gray in the winter and the really gorgeous mountain national parks (Mount Rainier, Mount St Helens, the Olympic National Park) are nearly entirely closed and snowbound for most of the winter. You can definitely see the cities and the coastal areas, but make sure to pack your umbrella.
