A couple of weeks ago I made day trip to Poznan from Berlin. There is a train leaving from the Ostbahnhoff station and arriving in Poznan glowny 3 hours later. Once I got there I started to walk towards the city center and on the way there I found a place selling Pizza that had an amazing margherita, yummy!
I visited the central square and I thought it was beautiful with the old buildings around it, including the renaissance old town hall. There were many ice cream places, bars and restaurants. In the information center they recommended me to visit Ostrów Tumski, an island between the two branches of the river Warta. This island used one of the main political centres in the early Polish state and now it's possible to visit the cathedral that houses some ruins from that era in the subterranean crypt. Very interesting city.