A dare
Go to a fast food place such as MacDonalds, place your order at the counter (not using self serve) but tell them you are going to the bathroom.
Go into the bathroom and strip off all your clothes then immediately begin masturbating. Continue for 2 minutes then dress quickly, do not wash your hands then return and collect your order.
Now sit and eat your meal.
Report in detail in this thread while still at the restaurant to enhance the humiliation.
Things to consider and options.
Select your outfit carefully, no underwear makes this easier, as does a dress.
Spend some time edging at home before going out.
If you drive, try edging in the car before going inside.
Have fun.