[Bart]

Girls or guys can do this.
For this dare you will need a pair of jeans you dont mind loseing or cutting, a dildo of some kind (suction cup might be useful) or something like a dildo.

You need to cut a hole in the seat or crotch of the jeans thats big enough for the dildo to go through.
Insert the dildo in your pussy or arse so the base is inside the jeans.
Now walk to a public place with a bench or seat. It can be as risky as you want it to be. Bus shelter/park bench somewhere like that.
Now move the base of the dildo through the hole and stick if it has a suction cup or put it on the bench or seat.
Now ride it the dildo for 20 minutes or until you orgasm.
If somebody comes along you can just sit on the dildo and they will never know.

If you cant hold the dildo in you while walking take it and insert it at the bench but that adds to the risk.