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; there's a free audioguide. Ferries to Pampus depart from Muiderslot port on a seasonal schedule; check the website for details. The trip takes 25 minutes.
You can also take a direct 40-minute ferry here from the eastern Amsterdam neighbourhood of IJBurg.