Wawel Castle is a huge and beautiful castle located right in downtown Krakow. One of the best things about this attraction is that you don't need to trek outside of the city to get there, you just stumble upon it. It was actually the very first UNESCO site in the world, which I thought was quite a cool fact. It is not known when exactly it was built, but it is estimated to have been started in the 13th century. You can spend all day wandering around here, so definitely set aside an afternoon to check this place out! Just be ready for crowds.