Any good restaurants open on Thanksgiving?
Check this one. Beautiful. [original link]
Whatever you choose, be sure to make reservations. It's a very busy day.
I suggest you check with the Interlaken Inn on whether they're serving on Thanksgiving. They are my family's next door neighbors and I have had many outstanding meals there. They have a very nice dining room or you can eat at the cozy bar.
You should try Paradox Lodge. A very nice place with French cuisine.
Last I heard, they had stopped running the restaurant and were only serving breakfast to their B&B guests.
Got that impression. Looks like a nice place.