A computer game, such as tetris, which looks like a spreadsheet. That way your boss thinks you are madly calculating your company financials when actually you are playing games.

//the numbers would be "falling" through the spreadsheet cells//

and thus transforming themselves on their downward journey. I'm now imagining some kind of horribly complex mathematical Tetris, which could only be played by Deep Blue and Stephen Hawking.

(metallic tones) "That's not a true prime! You can't put that there!"

(robotic voice) "Shut it, Steve. Do you want me to break out the chess set again?"

This could work. Early Tetris games used text mode graphics "blocks" which could just as well have been numbers. But is it possible to program Excel to run Tetris? I'd suggest it is.

Yea, I imagine.... the same way jutta imagins.
I would like Excell Halfbakery, where letters were translated to numbers using some algorythm all halfbakers knew..