The national museum of the Isle of Man, on Kingswood Grove in the centre of Douglas. The collection covers the full sweep of Manx history and culture, from the prehistoric Bronze Age hill forts and Viking settlements through to the 20th century. The Celtic cross collection is outstanding — the Isle of Man has some of the finest surviving Celtic stonework in the British Isles.
The museum is well-presented and free to enter, and a good rainy-day option if the Manx weather turns (which it will). The adjacent library holds the national archive of the island. Allow two hours for a thorough visit.