My daughter (then 3.5) became a citizen at 26 Federal Plaza, NYC. The ceremony was all for internationally adopted kids and was lovely–USCIS employees sang the Star Spangled Banner for us. No flags but I had bought some at a subway station candy/magazines/cigarette lol shop in the middle of the platform. Many happy memories. I rushed to get her application because the fee was going up from $85 to $115. I think it was 1999.
Of all the suggestions, I liked the mad money one–maybe some euros, yen and Canadian coins too.