Portland is a lovely city. I know I lived there 18 years!
The Old Port is a nice area to walk around, lots of shops and restaurants, that I am sure you will love. There is the Casco Bay Islands that has a Ferry line where you can sign up to cruise aroudn the small islands in the narbor and it is a lovely ride.
You can also walk arounbd downtwon protland, lots of history there, as well as the Porltand Museum of Art whcih has Winslow Homer paintings, quite lovely.
My mom runs the reservation department at the Eastland Park hotle across from the Museum, so I have lots of connections there if you need any other information