Have you guys played Sudoku yet? It is an addictive logic-based number puzzle.

"The attraction of the puzzle is that the completion rules are simple, yet the line of reasoning required to reach the completion may be difficult. Sudoku is recommended by some teachers as an exercise in logical reasoning. The level of difficulty of the puzzles can be selected to suit the audience. The puzzles are often available free from published sources and also may be custom-generated using software."

Here is a website that shows you a step by step process to create your own puzzle.