Operation Christmas Child is great, I've done it for years.
Wrapped shoebox christmas parcels are collected to give to children in orphanages and such across the world. Romania especially is very special to my heart after doing some youth work over there som eyears ago.
Essentially it's to give Christmas presents to children who wouldn't normally get anything.
One of the most heartbreaking stories I heard was when a load of presents were given out in an orphanage one Christmas and the children started banging them like drums as they didn't know what to do with them having not expected any presents.
There are guidelines as to what to put in. I usually go for some useful bits like a toothbrush, toothpaste, flannel, soap etc... usually funny kids ones then go for some toys and pretty things for the girls (you specify if for a girl or boy and what age range (I usually do one of each).
A hat, scarf and gloves and some warm socks if to a colder place.
Some fun stuff for Christmas day like balloons and streamers and party things.
Wilco's is great for it as lots of small fun stuff for kids.
It's a great thing to organise through work etc...
Site here...
http://www.operationchristmaschild.org.uk/