Southeastern Poland is an enigma to most visitors to Poland. Lacking an international "must-see" on the order of a Kraków or a Gdansk, most visitors from outside of Poland choose to give it a miss. And maybe that's the best reason of all to come. The region's hub, the city of Lublin, has a delightful Old Town that, while not quite on par with Kraków's main square, is nevertheless much more manageable and far less crowded. Lublin has a fascinating history at the confluence of Polish, Jewish, and Russian civilizations and maintains a lively cultural life, fueled by the presence of several universities and thousands of students.
Away from the big city, the smaller tourist towns of Zamosc and Kazimierz Dolny have always been popular with Poles and are only now beginning to attract outside visitors. The former is a nearly perfectly preserved example of Renaissance town planning. It's undergone a thorough facelift in the past few years and is a great spot to relax for a day or two. Kazimierz Dolny, astride the Vistula River, is one of those artsy towns that draws legions of gallery owners and antique shoppers, and wouldn't be out of place in upstate New York or Vermont. It's a laid-back weekend spot for stressed-out Varsovians and Lubliners to hike and bike, browse the galleries, and just hang out. In summer, the town maintains an active cultural calendar; don't be surprised if you run into a klezmer music festival or something similar while you're here. Chelm, an easy day trip from Lublin, is best known for its underground chalk mines, which you can tour and that make for an interesting diversion (especially for kids).
Travelers with a special interest in Jewish heritage will want to spend extra time in Lublin, which was once called the "Jerusalem of the Polish kingdom." World War II and the Nazi occupation put an effective end to Jewish life there, but city authorities are making an effort to reclaim at least part of that heritage and have laid out a walking tour of the major Jewish sights.
In Poland, you're never far from World War II, and eastern Poland was brutally affected. Just outside of Lublin stands the former Majdanek concentration camp that was once planned by the Nazis to be the biggest such holding camp in all of Europe. South of Lublin, the Bezec extermination camp was where the Nazis fine-tuned their "Final Solution." Both are now state museums.