Kraków is a great place to visit.
Costs are about 1/2 what they are in places like France and Spain on the Euro.
Most things are a short walk from the old market square, so stay near there if you can. As such, the old town it is full of old churches, gorgeous buildings, and a very large and vibrant town square lined with cafes. There are lots of classical music concerts. Visit St. Marys Cathedral and the other great churches in the old town. Walk up to Wawel hill and visit the Royal Cathedral and Palace. Take day trips to Oświęcim (Auschwitz) and to the Wieliczka Salt Mine, and perhaps to the Eagle-nest Castles in the mountains. Visit the Museums.
The most striking thing I remember from Kraków is the Kraków Hourly Trumpet Signal
Every hour a golden trumpet shows above Kraków's central Grand Square in the west window just below the spire of the higher, municipal tower of the St. Mary's Basilica. Then a characteristic trumpet signal, known and dear to every Pole, played by a live trumpet player, resounds all over the city's Old Town historical district. All of a sudden the melody comes to an abrupt end. The Kraków signal, or Hejnal Mariacki, dates back to the Middle Ages when it announced the opening and the closing of the city gates. The bugler also played it to alarm his fellow citizens whenever he saw a fire or enemy forces. And the melody's abrupt ending is said to commemorate a bugler shot through his throat by a Tatar archer in 1241.
My advice would be to rent a condo near the market square, inside the Planty. I had one last June, it was great, just a stone's throw from St. Mary's. The market square has several dozen outdoor cafe's, and there are hundreds of restaurants/bars within a block or two.
Drink lots of Zywiec.
Warsaw is worth a few days, for all of the WWII monuments, memorials, and memories, Getto museums and memories, plus the old town. It's a 4 hour train from Kraków.
Get the Rick Steves DVD on Eastern Europe, which covers these cities, plus others.
Prague is great, you can easily spend a week there.
Prague has the market square, charles bridge, and the castle hill with
St Vitus Cathedral and the palace a short tram ride away, or a longish
If you get a condo in the right place, you can walk to the major attractions in each city.
Contact me if you want more info.