Years ago, my wife and I went as the girl's room and the boy's room, respectively. Used some of that one line
redacting tape to make a grid of 4 inch tiles on a couple white T shirts. Then spray painted one light blue and the other one light pink. Removed the tape, and it really looked like tiles. Then we got out a couple different colored sharpies, and started writing graffiti on the "walls." All of mine was genuine graffiti I remembered from all the boys rooms growing up. Based on my wife's graffiti, girls' rooms don't have the raunchy stuff boys rooms do. It was easy, and people had a fun time reading the graffiti on our shirts. Good conversation item.