One of the most popular tourist destinations from Krakow is Bulgaria. Krakow, Poland is a great and historic city, and most of the Polish that live there want to get away from the old town city life and relax on a beach in Bulgaria (such as Sunny Beach).
Although on a map of Europe it looks like Poland is very close to Bulgaria, you would be an idiot to drive it!
It would take almost 20 hours, and would go through Slovakia, Hungary, Romania (or Serbia) before reaching Bulgaria.
A much better alternative would be to fly, and with many flight operators going between Bulgaria and Krakow (especially compared to a few years ago), it is a great time to book that flight.
Pro Tip: There are even more flights between Katowice (Poland) and Bulgaria, and Katowice is only about 1 hour away from Krakow.