Playdough has to go on the list for quiet activities. You can buy it or make your own quite cheaply. The link above is for the BBC recipe but there are lots of others available including ones that are actually meant to be eaten!
What's On Your Bum?
No link for this one because it's just a fab idea I read somewhere when my kids were small. Great for when the parent responsible for small children needs a lie down. Adult lies face down on a bed or sofa, children search for an item from around the house and put it on your bum. They say 'what's on your bum?' - which they love because, well, they're kids. You have to guess what it is. Try insisting they return it too for even less work.