'Au Calice' is popular with both locals and tourists. It's a fairly big place, so it might not suit your needs if you're looking for something 'small and intimate' but the decor's fine, the staff are helpful and the food's good & reasonably priced. (If you're feeling adventurous, they do an interesting local dish of pork chitterling sausage in a mustard sauce).
I'm almost certain that they'll be open on a Monday night in December but you could always phone first to check.
Au Calice
55, Boulevard Jacquard
62100
Calais
Tel. : +(33) 3 21 34 51 78
Chris