The official currency of Greece is the Euro (€ or EUR). Greece is part of the Eurozone and uses the same euro banknotes and coins as other participating countries.
Cards are widely accepted in hotels, restaurants, and tourist areas. It is highly recommended to also carry cash for small purchases at local markets, or while travelling on excursions.
ATMs are abundant in most locations.
The average temperature is 20°C/68°F for October and 16°C/60.8°F for November (https://www.timeanddate.com/weather/greece/athens/climate). There are about 8-10 rainy days per month, so you should expect rain on about one day in three.