A museum about the "vasa" warship - a vessel from 1628 which sunk on the first trip, and was recovered in the 1960s. The museum is quite impressive, and includes a lot of background information about the ship (as well as the whole ship, of course!) It is not possible to walk onto the ship, but the museum is definitely worth a visit (spent nearly 4 hours there) http://en.wikipedia.org/wiki/Vasa_(ship)