Well, MITA membership would open many island camping opportunities and is also a worthy cause regardless (OK, off soapbox now). There are a number of islands that are publicly owned. Those coming immediately to mind include Jewell in Casco Bay, Thief in Muscongus, Little Chebeague in Casco, etc. Jewell and Little Chebeague are apt to be pretty heavily used Labor Day weekend. Thief has just one useable site and is also likely to be busy. I believe Strawberry Island in Muscongus Bay is public and not too far a paddle from Bremen Cove Marine, a good put-in for Muscongus. There are quite a few other public islands so some research will likely be fruitful. Another alternative is Warren Island State Park in Penobscot Bay. It's a 2 mile paddle from Lincolnville Beach (can be choppy on a windy day) and has nice sites, fresh water, privy, etc. Check with Maine Sport for the tandem rental.... they're in Rockport, ME. Have fun!