convex: Just the green insurance card, registration, and the rental agreement. Make sure that Poland is included on the green card, and not crossed out. There's no green cards anymore within the EU - you just need the insurance documents. But you have to check - many rental companies are funny with what countries you can take a car to - it's not a given that they will automatically allow the car to be taken to Poland.
One thing to bear in mind when renting a car in the EU is that insurance policies can be different to what you expect from the US.
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