Off the coast of Muiden lies a derelict fort on the island of Pampus. This massive 19th-century bunker was a key member of the Stelling van Amsterdam (Defence Line of Amsterdam), a ring of 42 fortresses that were ultimately rendered useless by aerial warfare. It's now a Unesco World Heritage Site and is great fun to explore. Ferries to Pampus depart from Muiderslot port on a varying schedule in season; the trip takes 25 minutes.
You can also take a direct ferry here from the eastern Amsterdam neighbourhood of IJBurg.
Once on the island, there is the interactive Pampus Xperience, with audiovisual elements to bring the fort to life. There are also several games for kids.