It seems like not many people know that Harvard provides FREE historical tour, so here you go:
"The tour is around one hour long, and completely free of charge. There is no pre-registration for general visitors, however, if you have a group of 15 or more, reservations are required."
Free tour is available couple times a day and available whenever school office is opened.
here is the link to the Calendar:
http://www.harvard.edu/visitors/calendar
If you are interested in exploring on your own, tour Map and audio tour tracks are available as well.