I say all the usual stuff like thank you, how charming, etc. and I make a game of it for myself to see how I can get the item in someone take home pile without them knowing. I have even slipped the box/bag into their car in with their other items. Sometimes I receive a call saying I got someone's gift by mistake. I say 'oh no one has mentioned losing a gift to me. This works if you have a group of people that don't see each other a lot.