We finally did something I read about on Pinterest yesterday - GOOP made from cornflour. I already had some of the cloud sand (made from cornflour and baby oil) so we started with that and then moved on to the somewhat messier goop. I made the cloud sand over a year ago and stored it in a container with a lid and it was as good as the day I made it.
The kids had a great time throwing it around and then cleaning up afterwards. Next time though we will do it on the lawn and not the deck. I would also recommend you lock up and pets to avoid them getting covered and also eating the mess (might make for a sore tummy!). Head to my Pinterest board DIY kids stuff and activities (http://pinterest.com/anodonkeydesign/diy-kids-stuff-and-activities/) for some links on how to do this yourself.