OK, well it wasn't exactly my idea but that of my fantastic nanny.
DS had some boys round for a small tea party yesterday as one of his classmates was leaving - so 7 x 5 year old boys to entertain for a couple of hours. My nanny found an old roll of lining paper which was lying around at home (but apparently if you buy them they are really cheapo - less than a fiver). She unrolled a long section - about 15 feet long - and stuck it to a wall in the garden and put lots of crayons and pencils in plastic beakers for them. Seven boys then stood there like junior Banksys and drew rockets and aliens for over an hour with little or no intervention (and a couple of those boys are not renowned for their attention spans!)
When they were finished they each wrote their name alongside their bit and she took a quick photo of each of them against their bit, which they loved and I will either print up for them or email to their parents. (You could also cut up the picture into sections and let them take their section home, I guess).
I think this is going to be a mainstay of parties chez Anchovy over the next few years