I would like suggestions as to where to go after a September wedding in Poznan, I have been to Krakow, Prague, Berlin and would like ideas. I have a month free to travel.
A month is plenty of time to train to any neighboring country or fly to any far corner of Europe. See www.skyscanner.com for flights and Rick’s Train Travel Time & cost Map for an overview of train timing.
After the wedding you have a month to travel, a very envious position to be in. If you intend to stay in Poland to do some more traveling, I would heartily recommend Gdansk after Poznan or Torun if you prefer a smaller place. It's a direct connection from Poznan to Torun. I did this in 2005 coming from Berlin, had to transfer in Poznan for the train to Torun.. After that Gdansk is the place, see the towns in the lower Vistula area. Not all were damaged or destroyed in the war. Some survived the war intact, such as Chelmno northwest of Torun.
I invite you to Polish seaside with three cities Szczecin, Gdańsk and Gdynia and many towns as Świnoujście, Międzyzdroje, Kamień Pomorski, Wolin, Kołobrzeg, Koszalin, Darłowo, Słupsk, Hel, Sopot, Malbork, Elbląg and Frombork.
Out of that list above, I've seen Gdansk and Malbork, both well worth one's time in history and culture. Those I want to see especially historically among your suggestions are Frombork, Elblag, and Kolobrzeg along the Baltic.
Maybe more in Greater Poland? Poland Just Now will help with finding your itinerary as well as bookings.