Here's some of the ways I have done this this week...
Year 7 (angles lesson)
picture on board:
Year 8s and Year 10:
I gave them one of the www.mathsbox.org.uk 'settlers' target boards to work out
I gave them Dave Gale's QR Code Puzzle task (see http://reflectivemathsteacher.posterous.com/qr-code-puzzle-worksheet)
I found that by doing this I immediately had the attention of my classes as they entered the room and got sat down in their usual place. There was a quicker start to my lessons and I was able to still do all the admin like taking the register, checking h/ws etc whilst they were off and running on the tasks they were given or videos/pics they were to write down questions about. I will definitely continue to ensure that I utilise 'Meerkat Time'!