There are a TON of great activities right on property such at tennis, an 18-hole golf course, an amazing spa, horse back riding, and more. The link will direct you to the resort's activity page with information on booking.
If you're ready for an adventure, you can book the hotel's zip line experience at Bretton Woods. Groups should expect 3+ hours for the tour.
The Mount Washington Cog Railway is a mountain climbing train, which has been making its three-hour round trip to the summit of the highest peak in the Northeast (6288’) since 1869. You can buy tickets directly on the website.