Sometimes the challenge of getting to a place is half the fun. To get to Chebeague Island you have to drive to a parking lot in Cumberland, load your bikes onto a bus and ride the bus to a small ferry dock, then take the Chebeague Trasnportation Company ferry to the island. Kids really love ferry rides. If you prefer a more urban challenge, you can figure out how to park in Portland, then take a Casco Bay Lines ferry to the island.
Chebeague Island is a quiet community with just a couple small shops, a museum, a historic, and plenty of great views. There's an easy 6-mile loop road around the island with several cut-through options to shorten the trip. The small museum is worth a visit if you can catch them when they're open. We like to bike around, check out a couple beaches, and finish the outing with a Shirley Temple/beer at the Chebeague Island Inn. You can grab a table on the big wrap-around porch with a view of the bay, or head down to one of the Adirondack chairs dotting the rolling lawn.
Make sure you've got an updated version of the ferry schedule and get to the dock in plenty of time for your return trip. Biking Chebeague Island is a no-frills outing that makes you feel like you're miles and miles from everyday life.