The central and northwestern region is the new Poland, awaiting your discovery. "New" it is, but only in the sense of tourism exposure. It is, in fact, a very old historical hotspot. The Polish state was founded here in the Middle Ages. Within the region, Gniezno and Poznan's Cathedral Island are the main contenders vying for the title of the cradle of Catholic Poland. In this distinct landscape of historical rubble, churches, castles, and palaces, you'll also find the impressive Iron Age settlement of Biskupin.
Poznan is the place to start your exploration of the region. The city is unfairly brushed off as a stopover to get your business done as you hurry off elsewhere. That would be a mistake. The charming Old Market Square pulses with life force. The historical buildings and esoteric museums reflect the locals' pride in their history and identity. Moving in the northeasterly direction, you'll come to Torun, the birthplace of Copernicus and the Gothic center of the country. Take the time to meander among the red-brick towers and churches that survived the ages, then pause to munch on the gingerbread culture.
Outside of Poznan and Torun, the attractions are more spread out. It's easiest to see the area by car. To manage with just public transportation, you'll need to plan carefully.