There's a wealth of comedy scratched and written on and in public toilets, so let's be having the best you have come across.
And I am given to understand that some of the best and filthiest are to be seen in lady's loos, so come on girls lay some on us too. 
Here are some starters:-
In the cubicle........(an oldie from when it was only a penny)
"Here I sit broken hearted
Paid my penny and only farted."
At the urinal.........
"Please do not drop fag ends in the urinal as it makes them difficult to light"
On the condom machine..........(a play on the ice cream van slogan)
"Buy Me and Stop One"
