Looks innocuous, doesn't it?
Ackermann's function is one of the few things I actually remember from the recursive function theory course I took many long years ago. It's just a really neat function. Play with it a bit and you'll see what I mean.
Liven up a boring meeting. Instead of sitting there doodling, bring in a copy of Ackermann's function and see how far you can get with it. If you have ever played with numbers, I guarantee it will be a lot more interesting than drawing random designs.
Test your programming skills. There are a lot of short cuts you can find to help compute Ackermann's function much faster. How many of them can you find?
Make money fast. Bet a hotshot programmer that s/he can't write a program in less than an hour to compute A(5,5). Or be generous -- give them a week.