Coming to visit from WI...Whats the safest area?

We're coming to visit Boston & want to walk around & explore! We want to come & go without a guided tour, but also want to be safe when doing so. Looking for areas/neighborhoods that are the safest to walk around, even after dark. We want to stay close(ish) to Fenway park & also the downtown area, so nothing over the Charles river or by the airport. Thanks everyone!!