Traveling be train in Poland is fairly cheap and reliable. I would however advise you to opt for the 1st class train tickets as the cars are bigger and cleaner. I absolutely loved being in Krakow (even though I was born in Warszawa) and I highly reccomend going there. There are some areas which are very touristy and thus more expensive, but the city is amazing and the athmosphere is second to none. There are some really cool clubs and pubs in the old Jewish part of town and if I was you I'd make sure to go there at least once. There is also Wieliczka with the world famous salt mine and Zakopane as two main attractions close by in the area.
Hiking and camping in the Polish mountains is beautiful and adictive (of course make sure to have some good hiking boots as some trails are very rocky and steep). If you do your research you can even sleep a night at a youth hostel on the top of a mountain (in Polish look for "schronisko mlodziezowe"). Here is a link in Polish
to get you started and here is another with a bunch of accomodations in the Zakopane area
FYI - the main mountain ranges in Poland are: BESKIDY, BIESZCZADY, PIENINY, GÓRY SWIETOKRZYSKIE, SUDETY and TATRY. Also, seeing as Polish is a hard language here are a few key words when searching:
pension - pensjonat
mountain hostel - schronisko gorskie
guest room - kwatera prywatna, pokoj goscinny
summer house/cottage - dom letniskowy
pole namiotowe - campsite
Dworek, palac, willa - historical house, castle, mansion
Don't hesitate to ask any questions about Poland and I'll do my best to help you out.