Need ideas for a day trip in southern NH

My parents live in southern NH (Windham area) and I'm going to visit them for a quick trip later this week. I want to spend Friday doing something special with my dad for his birthday, but for the life of me I cannot find anything that is open that he hasn't already done (my folks have been to aaaaallll the museums). He's turning 80, so no hiking or outdoor sports. All the historic houses and artists' homes that I thought he'd find interesting are only seasonally open, so not until May. I'd love some creative suggestions of things we could do during the day (especially accounting for the potential rain).