Credit is owed to TheBOSman and CokeandTaco for finding a great fare from Portland to Boston on United Airlines. Originally only 3 CPM, the fare has since gone up but is still a great deal at 3.5 CPM. You can also do Seattle to Boston for slightly more cash and lots more availability.
Those in Seattle might benefit from trips to Fort Lauderdale or Orlando, which I’m sure are a lot more enjoyable for a winter mileage run. If you can still find them. The fare appears to be dead as I write this. At 3 CPM, it’s an amazing deal through EOS (end of service) given fare increases in 2012, but do watch out for direct flight numbers, as Tide_from_PAE warns. “Direct” flights are those that actually stop, and may even involve a plane change, but share the same flight number. United will award elite qualifying miles as though you flew without stopping.