Snooping Mr. Clumsy is an old school kid-friendly "reveal the image" Time Waster
As far as Flash games go, you can't get much simpler than Snooping Mr. Clumsy. It's basically a remake of one of those early 90's games where you had to gradually block off a level while avoiding one or more "baddies" that roam inside of the level and hit your line if you're not careful. The name of that particular game eludes me at the moment, but I'm sure you can help me out in the comments.
The main difference between the classic game and Snooping Mr. Clumsy is that, in the Flash version, the baddies roam all over the screen and not just inside of the space that remains from the level. To me, that makes it somewhat less fun, but it's also a bit easier that way.
The fact that the game has only four levels and is quite easy tells me that it might be aimed primarily at kids; if that's the case, they've done a very good job. While you may not be enthralled by this game yourself, if you've got a little tyke or two running around, Mr. Clumsy might keep them entertained for a while.