And another game for the next birthday
party:
Let your guests calculate Pi by playing some rounds of
“darts”.

Do this by targeting the blue shaped square on the above figure.
And then simply count the darts you are throwing and that will be
inside the square, and additionally the darts that miss the board
and hit inside the marked blue area.

And after a few rounds, let’s say 1.000-100.000 should be enough
to see some good results.
Now use this formula:

A small hint for the frustrated, invite people that are able to aim
the upper left square, but are not so much pros that they will
always hit the board only.