Are you going to Warsaw to visit? Because if so I would advise you to spend more time in Krakow than in Warsaw. Krakow is a MUCH nicer city, with much more to do and much more to city. Warsaw was burned to the ground in WWII, and though its restored town centre is nice, it has nothing a Krakow which was left untouched in WWII and retains all of its beauty and charm. The rest of Warsaw outside the old city and Lazienki park is rather ugly and lacking in personality IMHO.

As far as turning Krakow into a daytrip, it would be tough. Its about 3hrs away by train, 4 or so hours by car. You could book onto an intercity train in 1st class with assigned seats pretty cheaply, though i would definitely recommend at least spending the night in Krakow (the nightlife in the Rynek, town square is amazing, as is the Jewish quarter). From Krakow you could also make a jump to Zakopane, which is a gorgeous mountain town on the border with the Slovak Republic.