The Vasa sank on its maiden voyage in 1628 and sat in the harbour until 1961. The museum built around the recovered ship is Stockholm’s most visited, and the ship itself is the exhibit — almost intact, inside a purpose-built hall on Djurgården.
A timed ticket avoids the worst of the queue. Guided visits add the why-it-sank story, which the labels cover but a guide tells better. Combine it with Skansen or a harbour walk on the same island.