You can't fully appreciate coastal Maine without getting on a boat and exploring the islands. Sure, the beaches are great. Downtown Portland is great. But until you've gotten out onto the islands there is an entire world you're missing out on. Of all the islands you can visit, Jewell Island gives the most bang for your buck. Unfortunately, no public transportation exists to take you to Jewell. However, you can hire a water taxi or find a friendly local (your best bet is the water taxi).
Once you're on the island, the first thing to do is set up camp. Campsites on Jewell are first come first serve but there is usually only a problem on holiday weekends in the summer. Any day of the week you still want to get your pick of campsites. Most campsites are beautifully located overlooking the ocean and provide flat smooth areas to set up tent as well as a firepit and eating area. All camping is carry-in carry-out.
Also on the island are two WWII lookout towers. Climb up these towers and you're treated to a panoramic view of Casco Bay and a great sunrise/sunset.
You'll find other WWII ruins as well as "The Punch Bowl", hiking trails, and a beach where every rock seems to be perfect for skipping. This island is a true local treat! And much more fun than any hotel room.