I lived in Portland Maine from 2005 until 2008, kind of before it became super popular and gentrified (felt like that started around '08). Just an overall beautiful place to walk around. I like some of the random things you can find, random shops like Strange Maine (wonder if that is still around). Nickelodeon theaters was great, Bullmoose. South Portland has some relaxed beaches you can go to, Eastern Prom was always under rated IMO and is only getting better from what I hear and just walking around Old Port....There is a long walking trail that takes you past some train tracks along the water.
Deering Oaks Park (lived across from there for 3 years) is recommended just to chill, maybe get a sandwich and just enjoy a sunny day and relax a bit. Take the ferry to Peaks Island as well, awesome little town, rent a bike and ride around the island.