Anything else I should know?
bring flowers for your girlfriend's mother and some small flower of a box of chocolates for your gf...
don't worry about your broken Polish, Poles love when people try to speak their language and your gf is a huge support, so head up!
be polite...
if the father drinks vodka with you, you can't say no, but don't get too drunk so you won't do any crazy stupid things, or say anything you regret the next day...
other then that, good luck and enjoy the visit! :)