Warsaw has quite cool climate all year long. Summer and fall have mild temperatures with mostly sunny days. Daytime temperatue bein of about 15ºC - 19ºC (59ºF - 66,2ªF) but at night it may go down o 0ºC (32ºF), specially bu the end of fall. Winters are cold though temperatures are not fringe, snowing quite often though.
Zloty (PLN). Though in EU, it's currency isn't the €uro but Zloty. It's best to always carry some cash since credit cards are not accepted everywhere. Hotels, restaurants, some shops or even some taxis take them though. Just remember to ask for the exchange rate they'll charge! The best place to exchange are Kantor exchange offices. Some of them are even 24/7!