From Hagen it’s really easy and cheep to get to the capital of Poland. Wizzair.com is flying from the Airport of Dortmund (about 20 minutes with the car from Hagen) to Warsaw four times a week. Warsaw is a great to city with much to see and experience.
If you are there you should have a look on the Palace of Culture and Science, the Historic Centre of Warsaw with the Royal Castle Square (UNESCO World Heritage Site) and the Łazienki Park with Palace on the Water. And for a break I definitely recommend a cup of real chocolate in the chocoholics paradise Wedel ‚Pijalnia‘ in the street ul. Szpitalna 8.