Powerboats. The word says it all really. Given that typical stag weekends are supposed to be filled with exceptionally masculine activities, going on a powerboat trip is the ideal activity. Speeding along the Thames with a big engine on a boat will surely hit all the right buttons. Depending on which operator you go with, you should get a guide extolling interesting facts and stories about the famous London landmarks you pass while you race across the water, before finally upping the speed just for thrills.