Casco Bay Lines Mailboat tour is a 3 hour tour that stops to drop off mail and passengers at 4 islands. More details below: https://www.cascobaylines.com/maine-boat-tours/specialty-cruises/mailboat/
Portland Discovery Tours Harbor Lights and Sights Cruise has a narrated 90 minute tour of Casco Bay with views of 4 lighthouses. They have a shorter 60 minute tour also that's just of lighthouses. https://www.portlanddiscovery.com/charter-tours-portland-maine/#sthash.DJlViAb1.WmacX73w.dpbs
Take a 20 minute ferry ride from Portland to Peaks Island. Rent bikes or a golf cart and explore the island (it's about four miles around, but plenty of spots to stop and enjoy the views and take photos. If you go on Sunday, you have to hit up Reggae Sunday at Harbor View at Jones Landing. We went in July when we went up to Maine for our tasting, and it's a super fun day! https://www.cascobaylines.com/islands/peaks-island-maine/