If you want to stay comfy but look festive at the same time an "ugly sweater" is the most obvious choice. There are some not-that-ugly-christmas-sweaters out there. They're just simply cute and comfy and christmassy of course.
Another great option is a flanel shirt. You can wear it by itself buttoned up or just throw over any T-shirt you like and there you go. Effortless but still put-together.