Krakow is one of the oldest cities in Poland, and was the capital until 1609. Wawel Castle with Krakow Cathedral in it is built on some high ground on the banks of the River Vistula, with the Old Town just behind. Here there are several boats converted to bars and restaurants, and also some that do river trips.
It is undoubtedly one of the cleanest cities I have visited, although the Jewish Quarter is a bit run down. There are also very few beggars, and at least these try to provide some sort of entertainment.
Krakow is the ideal jumping off place to visit Auschwitz and the salt mine at Wieliczka. There are trips that visit both in a single day, well worth it.