I recently got back from a 3 week trip to Poland and had an amazing time much in part to the suggestions given by this forum. If I had to pick my top three, it would be Wrocław, Gdańsk, and Kraków (and an honorable mention for Toruń). The food in Poland is so good, you won’t be disappointed. Off the recommendation of Carlos from this forum, I ate at Pierogarnia Stary Młyn and they were easily the best pierogis I had in all of Poland. They are a chain and have restaurants in most of the major cities. I ate there in both Turuń and Wrocław and both were delicious. Also, Morskie Oko in Kraków is another fantastic restaurant. I took a pierogi making class while I was in Kraków and it was a really fun experience and I highly recommend.
There are many day trips you can take from Kraków including Auschwitz, the salt mines, Zapilie, or Tarnow, but there is plenty to do in the city to keep you occupied for days. I got a private tour for a very reasonable price and I cannot recommend her enough. Her name is Maja and she studied architecture. She knew so much about the city from history to food to lore and was extremely engaging the whole time. I could pass along her info if you are interested.
From Wrocław I took a day trip to Książ Castle, again because of info given by this group. I just suggest starting your day early so you have time to see everything. The Palm house was much more fascinating than I expected but I ran out of time to see it all.
If you go to Gdańsk, going to Melbork castle is a must. If you don’t mind a long day trip, you could visit Toruń. It’s a cute medieval town that can easily be seen in a day. While I was in Gdańsk, everyone told me I needed to take a trip to Sopot. I didn’t have time to make it out there but everyone assured me it was worth it. Not sure if it would be the same in fall though.
Many people say Warsaw is a stopover city, but I really enjoyed it. It’s a vibrant, modern city with beautiful parks and delicious food halls. If you end up in Warsaw, you have to visit the university library gardens. It’s a marvelous sight and gives you the best views over the city.