Conways Game of Life
A long time ago, some 10 years maybe, this was a contribution to a programming contest for a computer magazine. The contest was based on writing a Conway Game of Life (wikipedia)implementation in Excel. Open the Excel document, enable macro's and click 'Start Life' to see the evolution grow. Almost like a real computer animation. You can even design your own pattern and start an evolution by filling the Excel cells with a background color, just select 'Nieuwe Eigen Levensvorm' and click Start Life, draw your pattern in the grid, select 'start' and click 'Start Life' again!
Another contest was based around the old 'knapsack problem', see wikipedia. For example you have different lengths of timber, and you have to find the optimum solution for obtaining a certain length by maximizing the use of a standard length. This Excel document will calculate all possible combinations and show the 5 best solutions!