Go to any grocery store and grab a lemon-lime gatorade or powerade sports drink, anything with a clear bottle. Sneak it into the bathroom stall, open and drink it down to make room in the bottle. Piss into the bottle to fill it the rest of the way, then buy it in a checkout lane. (be certain the clerk is an adult of course.) Drink from it on your way out of the store and finish it, it'll fill your bladder again for your next dare
Also optional is doing the same thing at a gas station with a clear cup from the soda fountain, sneak it into the bathroom and empty your bladder into it before filling the cup with a drink. Drink it on your way out of the store and finish it, you get double points for doing both.