Admission is included in the London Pass and is free for children.
The HMS Belfast is a WWII warship, England's most powerful of the time. You get to walk through many sections of the ship. See what it was like for sailors and officers. Stand on the bridge next to the Captain's chair. Aim the anti-aircraft gun (you can climb into the gunner's seat and actually move the turret and adjust the position of the barrel). Walk through the engine room and learn how steam engines worked. See what a naval kitchen was like in WWII. There's plenty to see on this ship.
The ship is full of tight, steep ladders. Keep this in mind before you go here.
The HMS Belfast is a few minutes walk from the London Dungeon and is about a 5 minute walk from the Tower Bridge, on the opposite side of the Thames from the Tower of London.