A game of Countdown is a great way to start or end a lesson.
You can download an excellent free program for the Numbers Game from Chris Farmer’s CSF Software site.
For a low-tech solution print out the Make a number puzzles with solutions from Brain Food.
For other number puzzles see the Number, one of the Puzzles and Games series of pages.
For a new challenge why not try Coundown with Fractions from Nrich? See also the Nrich Countdown collection.