If you prefer museums and galleries - Krakow and Warsaw have dozens of them. 2-3 days for Warsaw will be enough, in Krakow you can spend even a week without getting bored (don't miss Auschwitz and Wieliczka salt mine nearby). Wroclaw, Poznan, Torun and Gdansk are also interesting. It can take you around 2 weeks to explore all these cities, if you have more time left you should pick something from the list WildRoo posted above.
There are many lovely small towns as well - like Kazimierz Dolny, Sandomierz or Zamosc, but it can be problematic to get there in a reasonable time using public transport.
Trains are the most convenient way to travel in Poland - they are slow but inexpensive.