Go jogging naked in public, preferably along the main city avenue or in a similar, popular place. You could steal and hide the birthday guy's clothes to make him do it together with others.
I had the idea to do this myself as a stepping out of your comfort zone challenge but frankly didn't have enough guts to do it alone (maybe for the better). A company, especially a birthday party one, on the other hand, is a different story. Perhaps less chance to get arrested.
Wear sneakers and sports headbands only, to make it understandable that you run a "marathon". The birthday guy could wear a headband saying "40!" or something like that, to make him stand out. The group could also carry a long band in their hands, stretched over their heads, while running, with "Happy 40th birthday [his name]!" or a similar phrase written on it, to make the people around understand what's going on.
It's better to do this in daylight for a bigger challenge and to gain some reactions from public. Early morning is an option if you don't want to attract too much attention. Maybe it could be your morning hangover run for one of the days finished by a jump into a lake or something like that:)