Peaks island is the most accessible island in casco bay. The ferries run constantly and it's only a lovely 15 minute ride. Once there, you can walk the entire island in less than an hour or rent a bike and ride the loop road. Beautiful views of the other islands in the bay. There are a couple of shops near the dock and the Inn at Peaks (top of the hill when getting off the ferry) has a nice restaurant and bar. Sunday afternoon at Jones Landing is famous for it's "reggae sunday" where the music and beer both are flowing! It's not to be missed when visiting Portland.

More info at www.peaksisland.com