At KRK, stay airside—small terminal with limited time buffer makes staying near the gate best.
Take the train to Krakow Glowny (17 minutes, PLN 10). Walk to Rynek Glowny, the Main Market Square, in 10 minutes. Spend 45 minutes in the square and St. Mary's Basilica (PLN 15 entry). Walk south 10 minutes to Wawel Hill for views of the castle exterior. Head back to the station with 40 minutes to spare.
Follow the same route as the 3-6 hour plan, but add the Wawel Castle interior (PLN 35, about 1.5 hours). For lunch, Restauracja Miod Malina on Grodzka Street does Polish dumplings and mains for PLN 35 to 55. Walk through the Jewish quarter of Kazimierz for 30 minutes before heading back.
With 10 hours, add Schindler's Factory Museum (PLN 27, 2 hours, tram 3 from the main station) or a half-day trip to the Wieliczka Salt Mine, 13 km southeast and accessible by minibus in 30 minutes for PLN 10. The mine tour takes 2 to 3 hours.