Eastern Cemetery and Evergreen Cemetery are well worth the visit. They both offered guided tours in the warmer months, and feature interesting gravestones and burials. The Portland Observatory is close to Eastern Cemetery as well. On Peaks Island, the 5th Maine Museum and Battery Steele are nice visits. I haven't been in a while, but read an interesting article about Fort Western in Augusta. Fort Knox is a cool visit, and if you're in the area the Buck Monument is more a curious roadside attraction. Joshua Chamberlain's house in Brunswick is a nice shrine museum to him and his accomplishments. I'm happy to provide any other recommendations if there are periods of time or specific areas of interest you're particularly intrigued by.