One of these is the infamous Ft.Gorges, a fort that was built during the civil war era to support surrounding forts on the entrance of the Fore river and the Portland-South Portland harbors. The structure took so long to complete that by the time they finished, the cannons and fort itself were obsolete. One lone cannon was left at the top of the southern spire if you get a chance to visit.
Another option is Jewel Island. An infamous island in Casco Bay (rumored to house buried treasure from Captain Kidd), Jewel Island is a common place for day cruises and cocktail parties from local pleasure boats. The island is heavily wooded and has a very jagged/rocky shoreline, which wouldn't make for the most comfortable camping conditions....
In terms of camping, there is a much more friendly island accessible via sandbar at low tide from "Great Chebeague". Little Chebeague used to be a training facility for fire fighters training in the Portland area, and recreational facilities for stationed naval members during WW2. It is much flatter, has plenty of beach front, and makes for great picnicking and camping.
It was once rumored that Casco Bay had an island for every day of the year (in reality there are somewhere around 280 I believe). This comes from a mixture of publicly and privately owned islands, which are mostly inaccessible without a small vessel of your own. The entire Bay is littered by beautiful rocky coast including a town with the most shoreline in the continental U.S. If you are looking for a reason to come and visit Maine this summer, there are at least 280 in these islands, their wildlife, and their sandy beaches along the Atlantic coast.