Willamette Valley - or the coast
Living in Portland, I prefer the valley rivers (the Sandy, Clackamas, and North and South Santiams), but the coastal rivers are pretty interesting. I'm not experienced enough with the coastal rivers to make a suggestion, but both the Sandy and the Clack generally fish well in December. We tend to go where the conditions are best. The coastal streams respond fastest to rain events, but also recover very fast. If it is raining a lot, try the valley rivers. If it has rained recently but let up for 24-48 hours, the coastal streams would be prime. So my advise is to leave your venue open until you arrive - then call the shops and get the lo down.