I prebuy about $100 of local currency before I leave on a trip. When I returned I went back to sell some back to them, and they would only take the paper currency! Even though originally they gave me a mix of paper and coins. Strange logic.
Also be careful that you don't get rid of your local currency too soon. Suppose you've spent all your euros, settled into your plane seat and then they announce that the plane is broken and you have to go back into the terminal to wait for another one! Now there's no way to buy just a bottle of water, or a candy bar, or an apple. Back in the day the flight crew used to walk around with a bag once they were airborne soliciting coins and spare change for UNICEF. There were also deposit stations inside the terminals. A great idea.