Back from 14 days in Poland. I've been to Krakow, Warsaw, Gdansk, Rzeszow and some smaller villages around Poland.
Here is what I would recommend:
Krakow is the highlight. You can easily spend 3-4 days there to see the highlights. Consider taking a bus to Auschwitz and Zakopane, even it is for just the day. Spend one day in the Jewish area of Krakow (Kazimierz), spend one seeing the highlights of Old Town Krakow- Wawel Castle, St. Mery's church, walking around the Rynek and Cloth Hall, etc. Go to a milk bar for a unique eating experience. We liked the one on Grodzka on the way to Wawel. ) Milk Bar Termida?). Walk the Planty and just get lost down some side streets.
If you have an apartment, consider shopping at Stary Kleparz which is a local food market dating back many centuries.. You can get anything there.
Hire a driver to take you for a combo trip to the Salt mines and the ethnographic museum nearby. We also went to the Museum of the Home Army which is outstanding, but may not be of interest to the casual traveler who isn't interested in the history of WWII.
Gdansk is our second favorite city, although it is similar to Copenhagen. You can easily spend a day walking around there and also consider going to their fabulous new WWII Museum.
Malbork is a nice train ride from Gdansk. Just find the train station and ask for help. We hired a taxi to drive us from the train station to the castle. We did a LOT of walking that day- the place is massive and very impressive!
Warsaw is our third favorite. Their old town is lovely, but I couldn't get over the fact that it was essentially mostly rebuilt and not truly "old." Poland certainly was ravaged by war and if you are interested in the WWII history of Poland, there are several outstanding museums.
We have visited the villages of our grandparents after finding long lost relatives. We stayed with them this time and did local research for a book I am writing. That was truly the highlight of this past trip. I wish everyone could find a village in Poland to visit to see the countryside.
We also went to the Przeclaw and Lancut castles. Wow! There are so many wonderful things to see.
Try to go to a skansen or ethnographic outdoor museum to see how people live 150-300 years ago. They are real treasures.
Poland is not only beautiful with much history, but is very inexpensive.