Weather wise, the best time to visit southern Poland would be late Spring - early Summer, however early September should be fine too.
If you are not too keen on touring concentration camps, know that Krakow has a number of other interesting and exciting day trips too. Here are some of my favorites in no particular order:
Wieliczka Salt Mine - really amazing subterranean architecture made of salt in this ancient mine, not too good place if you are claustrophobic.
Zakopane - resort spa town makes a great base to explore and hike the impressive and stunning Tatra mountain range.
Wadowice - the small town where Pope John Paul II was from, nice baroque old center, make sure to try the creame pastries the town is known for.
Tarnow - a gem of a town, off the tourist radar, with much Polish medieval and renaissance architecture eye candy.
Pszczyna Palace - another off the beaten path site, a grand baroque palace estate of the former German Duchy of Pless, the "Versailles of Silesia".