I plan to go to Poland soon and I might need a rental car.
What are the documents needed to drive in Poland as a European? Is my national driving license enough?
You should get a European Driving license. In my locale (France), every new license emitted since 2013 is automatically a european license. If you have an old license, simply asking the relevant authorities in charge of license delivering for renewal entitles you for an upgrade to the european format.
If you don't have a license that follows this format, according to this source, the acceptation of your license is country dependant.
EDIT: If you want to see the exact list of the countries participating in the EUropean Driving license, I invite you to read the article. According to this source, if you don't have such a license, you'll require an International Driving Permit delivered in your home country.