You can sign up for free museum passes at most branches of the Boston Public Library. That includes Children's Museum (discounted to $2 per person, up to 6), MFA, Harvard Museum of Natural History, Museum of Science, New England Aquarium (Sept-June), and the Franklin and Stone Zoos, among others. You just have to have a valid library card and children must be accompanied by an adult. It helps to sign up for a date in advance. Check the web site for details - http://www.bpl.org/general/circulation/museum_passes.htm