What are some good tourist sites on The Isle of Man?

There are some great historical sites on the island eg. Castle Rushen and Peel Castle. There is also the famous Laxey Wheel and Calf of Man. If you're into nature, there are some wonderful glens that can be explored on foot.
I would suggest a good place to start is the Manx Museum - you can find out about the history of the island there.