Add the ability to use diagonals in your puzzles. (Hint: All you'll need to do is combine the formulas I've already established. You don't need any new ones.)
Create a game of BattleShip for two players on the same computer. The game will print out a grid. (Pre-set the location of the fleets to make it easier.) Let the user choose a location on the grid with a checkbox. Report the result of his firing back, and then give the screen to the second user.
Write a version of Conway's Life. This program simulates cellular life on a grid with three simple rules.
Each cell with exactly three neighbors will become or remain alive.
Each cell currently alive with exactly two neighbors remains alive.
All other cells will die off.
Randomly generate the first cell and let the user press a button to generate the next generation.