We spent nine days in Warsaw and seven in Krakow and didn't tire of either place. Can you work at least one more day into this layover?
I think to appreciate Warsaw you really need to educate yourself about the history, particularly WWII. Don't miss the Historical Museum of Warsaw which includes a movie showing footage the Nazis took on their way out, blowing up the city as they went. Also don't miss Wilanow Palace and Lazienki Park. Lots of people, especially the 35-and under crowd speak English so try to talk with them. We found them very friendly and even managed to get ourselves invited to the University Choir's end-of-school year BBQ. As for food, people either love or hate Eastern European fare, personally I love pierogies and everything that goes with them. It is heavy on the meat too.
I took a "tour" from Krakow to Auschwitz, booked through the TI, which amounted to a semi-private taxi drive there and back. Keep in mind if you book some kind of "big bus" tour to Auschwitz there is a growing dislike for that format -- too many people do not treat the experience as it should be treated, its just the next stop on their day trip (they often do the salt mines in half day and Auschwitz the other half). And to be honest as much as I wanted to see Auschwitz, and recommend you go if it is important to you, keep in mind it ruins your day in a way because its difficult to move on to anything joyous after that. So don't count on doing much else that day.
Enjoy Poland, its wonderful!