I attended the Summer School at UMCS Lublin in 2009, it takes 1 month and includes language, culture, music and even some field trips. And it is cheap!
cjkp.umcs.lublin.pl/eng/
I've studied twice in this one during the summer. It was easy to go to class, but tough to do any work at home because my classmates and I hung out at the beautiful beaches in Sopot the rest of the day. I would recommend it, but be careful of the accommodation they put you in. I was lucky and stayed in a house with this very sweet older lady, but some people were in these awful concrete housing blocks. Try to swing a combination of a home that is in or close to Sopot. I stayed in Gdansk Oliwa, which was just two or three stops on the local commuter train.
Powodzenie
Oops I forgot to post the link.
ssp.edu.pl