My souvenir of choice when I visited the States - baseballs!
It all started when my family and I went to watch the San Diego Padres play the Dodgers at Petco Park. At about the bottom of the fourth there was a break and one of the Padre mascots started shooting foam baseballs out of a giant bat into the crowd. We were way up in the cheap seats and a ball was fired to the right of where we were sitting. However the wind caught it and it was pretty obvious it was heading our way.... I stuck out my left hand and the ball stuck - caught it!! And where were my wife and kids? They were off getting hot dogs and missed the whole thing!!!
Cheers,
Mario