You really can't experience Portland without experiencing the islands of Casco Bay. The bay is everything to the city. It drives the fishing industry, the tourism industry, and most of the local fun. A ferry service leaving off Commercial Street in Portland will take you out to most of the major islands at any time of year. The most popular island to visit is Peaks Island. A quick ferry ride drops you off right down the hill from a locally made ice cream shop as well as a bike rental shop.
Bring your own bike or rent one from the shop. Biking is the best way to see all of the island in a day. A loop road goes all the way around the island - it's around 4 miles - with lots of other side streets to explore.
You'll want to schedule in time to eat at one of the restaurants on Peaks or pack a lunch to eat on the beach. On Sundays during the summer get out early (first thing in the morning!) to make sure you can get into Reggae Sunday at Jones Landing. It's the biggest party in Portland.
Peaks Island is a great day long get away for people of any age or physical ability. You can bike, walk, eat, lounge, explore. For the quirkier traveler the island also boasts the world largest umbrella cover museum.