Saved from the wrecking ball in the 1990s and lovingly restored by the community, Highfield Hall dates from 1878 and was one of the early summer mansions to grace Cape Cod. It's now open to the public as a museum and cultural center; check the website for events and activities (art exhibitions, concerts on the lawn etc). Beebe Woods, the lovely surrounding gardens and trails, is open to visitors year-round.
In addition to its regular hours, the hall is open for free from 4 pm to 7pm on Fridays in July and August. There is also a short holiday schedule from roughly Thanksgiving to the first week of December (check website to confirm dates).