I have volunteered in a charity shop for about 5 and a half years. So many memorable donations, and not just at Christmas.
A folder with old travel tickets, itineraries, hotel receipts etc and also with about 50 envelopes of assorted foreign currency - took them to the bank and got about £550.
A Victorian copper jelly mold which looked very grotty and sold for £175 at auction.
Loads and loads of gold/silver jewellery which always sells well. (even single earrings and broken pieces which sell for scrap)
My theory is that the best stuff comes in plain black bin bags. So often I'll rip open a bin bag of clothes and it'll be full of Hobbs, Reiss, Phase Eight, Cos, Karen Millen or Boden. The fancy designer carrier bags generally have the Primark and H&M.
In terms of awful donations... yes to the people who just tip all their junk into a bag and let us deal with it. We do not want dolls with their heads missing, empty DVD boxes, your "Gucci" handbag bought for 3 euros in Greece or chipped mugs. Thank you. We do see our fair share of fairly minging pillows or stained clothes but those just go straight into the rag bag for recycling. I've never found sanpro (used or otherwise) or nappies.
But please please please anyone planning a post-Christmas clearout just ask yourself "Would I pay money for this?" if the answer is probably not, then bin it.