I would recommend the Westin Waltham. It is a great hotel, especially for the $89.00 weekend special rate. As mentioned before, it is off of Rt 128, so access to Foxboro is quite easy.

Another possible option would be to fly to PVD. It is about the same distance from Foxboro, but because of Southwest's presence, airfares are usually lower than flying to BOS. There is also a Westin in Providence.