This is a simple but very popular game of skill (and a bit of luck) that the children always ask to play at our parties, it also provides good photo opportunities! Things DO get a bit messy so you might want to play this in an area which is easy to clean or protect by laying down some old towels, etc. You also might want a spare towel or two for players to help them clean themselves.
On a large plate, build up a large lightly compacted mound/castle of flour. Give it a flat top and lay a small coin on top of this. Using a blunt butter knife or similar, players take it, in turn, to remove sections of flour away from around the coin (ie each player removes a section/slice of flour each turn). If a person on their turn causes the coin to fall into the flour they have to pick the coin out using their mouth only!
The game can be played for several rounds, rebuilding the pile of flour each time.