Too late for this year....but Jonathan's on Long Island turned out to be a great spot. Prix fixe for Thanksgiving with all the trimmings -- or order off their menu. They had a crush of people but managed to honor reservation times, not rush anyone and provide attentive service. I'd go back!
i don't know the price point for benchmark in park slope but their cooking is generally very good and it would be a lovely thanksgiving. i think 12th street bar and grill usually does thanksgiving also.