Kraków is an amazing city to visit all year round. In the summer the weather is beautiful (and can reach as high as 35C), and you can go and enjoy the sun in the main square, or check out the Castle and the great views of the river.
Or in Winter you can check out the Christmas markets and head down to Zakopane (nice mountain range 2hr coach journey away).
Check out our Weather in Krakow page too!
Read about exploring the abandoned Liban Quarry (and ex Nazi labour work camp)