After giving the speech at Gobbler's Knob (whether good or bad) there is an option for you to run the T-shirt giveaway. Move to the T-shirt cannon and pick it up. You need to use one hand to cock the gun before each shot and the other to pull the trigger. There are bunches of balloons around the area and you have to shoot all of them. These include the ones on top of the gazebo, those in people's hands, those that are sort of behind you. Some will end up partially behind barriers so can be tricky to hit. There is a time limit before the scene ends, so if it runs out, you just have to restart and try again. Shouldn't take more than one or two attempts to get all of them though.