Poland in 2-3 days
Fly into Krakow, spend one day at Auschwitz, one day checking out Krakow and on the third day, go to the Wieliczka Salt Mine.
In 7-9 days
As above, on day four take the train to Warsaw (three hours), on day five take a train to Malbork, on day six take a train to Gdansk for a two-day stay.
Two weeks +
Spend the first three days in Krakow, then take a train to Wroclaw for two days. Take a train to Warsaw for a day then catch a train to Bialowieza to visit the Bialowieza National Park.
Spend two days there then take a bus to the Great Masurian Lakes and stay for another two days.
Take a bus or train to Malbork to visit Malbork Castle then a train on to Gdansk and take a few days to explore the Polish Baltic Coast.