We're heading to Boston in December, when all (?) of the walking tours are on hiatus over winter. Are there any good self-guided tours of Boston or Harvard, especially with a focus on architecture and/or history?
Probably, the Freedom Trail is the best known walking tour in Boston. It is a 2.5 mile walk. You can book a tour, but you can also easily download the map and follow the trail by yourself.
Boston has a lot of great self-guided tour options. The Freedom Trail is obviously the most common and popular. There is also the Black Heritage Trail that is a self-guided tour of the Beacon Hill neighborhood with a lot of history. Beacon Hill has a tremendous amount of 19th century architecture. The Irish Heritage is another viable option. You can find info on all of these online.
If you're open to MP3 tours, Audissey Guides has a couple of free tours of the Boston Common and the Harbor Walk. Also IWalked Audio Tours has approximately eight Boston-based tours of various neighborhoods that highlights history and architecture.
Enjoy your travels!