Dandyman
In London village.
Gdansk's pleasant enough, although there's not that much to do and it always seems to be bloody FREEZING there. Malbork's about an hour south by train, if whopping great castles are your thing, and Torun's further south, although I'm not sure it'd be worth a three-hour trip. Probably worth getting the local train along the coast to Sopot for a bit of bracing seasidiness.
Malbork castle is HUGH.