It's kind of a drive from Alton Bay but The Manor on Golden Pond in Holderness is perfect for a special occasion. Other than the regular menu they have a great 8 or 10 course meal for two that is literally soup to nuts. If you warn them in advance, and they usually ask when you make the reservations, let them know it's an anniversary and they will make up special menus with the couples name and date on it. They make every effort to make it as fine a dining experience as you will find in the lakes region. I wouldn't go there every day but it is a really cool place for a special dinner.
http://www.manorongoldenpond.com/pages/fineDining.html