Poland Copernicus Science Centre

This Science Museum, built in 2010, is a great place to visit (not only with kids), but also a nice object for architecture photographers. It looks especially good at evenings. In the warm months you can walk up and visit the roof (with its gardens and a view on the riverside and the city center).
Bear in mind, that the place is very crowded when it gets warm, as the nearby boulevards are a very popular place for walking and partying. It is much better in the late autumn.

The best is to reach the place by public transport, especially the subway.
You can park your car nearby but on weekdays till 18:00 you have to pay for parking on the streets, and sometimes it's difficult to find an empty place.