Luke P.
Google
One of the better museums I've visited, this ex-World War 2 submarine allows you to take a peek into what used to be part of the Finnish naval force and let's you experience the insides of a submarine. Even though the sub is not really big, it is quite informative and really gives you an idea of being a submarine sailor.
They're only open during the summer months, and I highly recommend this if you get a chance to visit. You'll need to pay for the entrance ticket, which entitles you entry to the War Museum on the island as well.