Krakow is amazingly beautiful and interesting town, so you may end up spending all your time in the town itself, but there are some great opportunities for hiking nearby.
There are several websites about Krakow that you can look at. This one lists some ideas for day trips. The Ojcow national park is just north of Krakow, apparently it is possible to get there by bus. If you are willing to travel a bit further, about 70km south of Krakow are the Beskidy mountains, with Gorczanski and Babigorski national parks. All of those are definitely worth seeing, and offer plenty of hiking opportunities.
It takes about hour and 30 minutes to get from Krakow Main station to Sucha Beskidzka, which is sort of a gateway into that part of Beskidy mountains. There is a very nice brochure(pdf) describing some hiking opportunities in the region.