Rugen
The island of Rugen, off the northern coast of Germany, is just delightful in summer. There's lots of outdoor activities, and it's an easy landscape for biking and strolling in the woods or playing in on the beach. The towns are kid-friendly and accommodations are set up for families. It's the perfect place to relax.
Jagdschloss (Granitz) - The Jagdschloss, a 19th century hunting castle, is not some rustic little retreat. Rosy pink crenelated towers rise above a two story mansion - hunting in a luxurious setting. Inside, the downstairs rooms have an incredible collection, row and after row, of elk, deer, big horn sheep skulls and antlers, wild boar tusks, a majestic stuffed stag, and even tables and chairs made out antlers.
It's a pleasant walk from Binz to the castle, but you can also pick up the cute tram (Jagdschloss Express) in the center of Binz, it goes directly to the Jagdschloss.
Rasender Roland - The Rasender Roland is a pint-size narrow gauge steam train, always filled with families on a day outing. Little kids will enjoy standing on the platform, watching the black and red engine puff into the station. The train goes from Putbus (west of Binz) to Gohren, with stops at the Jagdscholoss, Sellin and Baabe along the way.
Eisenbahn and Technik Museum Rugen(Rugen Railway and Technical Museum) (Prora) - This isn't a big gleaming technical museum, but it has a few gems, especially from the East German era. See an original "Trabi," fiberglass cars made in East Germany or a hulking big WWII locomotive.