Gather some interesting problems, print out copies, provide paper and pencils, and step back and watch the wheels turn! You may want to meet in a room with a black- or whiteboard, so that students can share innovative solutions.

Some sources of good problems: 

MAA’s Minute Math:  Quick problems and interactive solutions from the MAA's American Mathematics Competitions. Hint: the easiest problems appear on Mondays, the hardest on Fridays.

Mike’s Logic and Math Problems

Mike works in the Office of the Chief Actuary of the Social Security Administration.

Mathematical Quickies: 270 Stimulating Problems with Solutions, Charles W. Trigg, Courier Dover Publications, 1985.

"Nearly 300 mathematical brain-teasers from the fields of arithmetic, algebra, plane and solid geometry, trigonometry, number theory, and general recreational mathematics." 

Solve This:  Math Activities for Students and Clubs, James Tanton, MAA, 2001.

An example from Laundry Math: Can you transfer the T-shirt from one person to another?