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!
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.
Oops I forgot to post the link.