Well, seeing at Christmas is a Tuesday and a lot of people will take the Monday off, I expect DC traffic to get crazy starting on Friday Dec 21.
The drive down the eastern shore should be less stressful. The Hilton Garden Inn on Kent Island, is nice and is about $109 a night. From there, it should be about 1.5 hrs to National harbor. Island should be lit up nice for Christmas.
On the other side of the Baybridge is Annapolis, and the Double tree is $88 a night. From there, it should be about 1hr to National harbor. The "Old town" section should be lit up nice for Christmas.