The big swath of territory that lies southwest of Warsaw and between the popular cities of Wrocaw and Kraków is mostly, unfortunately, unvisited by visitors to Poland. Lódz (pronounced woodge), Poland's second-largest city, is a 19th-century boom town that, like parts of the industrial Midwest of the United States or the Midlands in the U.K., fell on hard times in the modern era and has had to reinvent itself. The results, so far, have been mostly positive, but the city hasn't quite yet broken into the top tier of tourist destinations.
Farther south, the Upper Silesian heartland (not to be confused with Lower Silesia near Wrocaw) has long been Poland's main industrialized region. The smokestack-laden megalopolis centered on Katowice and including the cities of Bytom and Gliwice is home to some 3 million people but largely devoid of traditional tourist sites.
Still, there are several good reasons you may want to schedule a stop in either Lódz or Katowice (or both). Lódz -- sometimes called HollyLódz (pronounced Hollywoodge) -- is home to Poland's famed film industry and was the early stomping ground for Poland's trilogy of world-leading film directors: Andrzej Wajda, Krzysztof Kieslowski, and Roman Polanski. It was also the site of Poland's second-largest wartime Jewish ghetto, after Warsaw, and parts of the former ghetto still look very much as they did during World War II. If you're interested in Jewish or Holocaust history, you can walk the former streets and take in the story of the Lódz ghetto at your own pace. It's deeply moving in a way that more highly polished memorials or museums often are not.
Katowice is an important transportation hub, lying on the main rail line between Prague and Kraków, and the highway between Wrocaw and Kraków. It's got some great restaurants and some offbeat attractions that can feel like a much-needed antidote to sometimes overly touristy Kraków.
The pilgrimage city of Czestochowa is arguably the region's only genuine must-see. The Jasna Góra Monastery, home to the fabled painting of the "Black Madonna," has drawn believers and miracle-seekers for centuries, and retains an aura of hushed holiness into the modern age. History buffs may want to push farther south to the city of Cieszyn, which straddles the border between Poland and the Czech Republic. This once-independent duchy in the Middle Ages proved a sore point between Poland and then-Czechoslovakia in the run-up to World War II, when Poland forcibly annexed territory on both sides of the border just as Adolf Hitler was making his own Czech land grab. Polish-Czech relations are much improved since then, and you can leisurely stroll both sides of the border by crossing a small footbridge over the charming Olza River.