maybe some public action like... x and y in a live concert during the night, in an outdoor festival of some kind and x just pulls y to a dark corner and hopes theyre not visible so x just slide some fingers in y's pants and make the best out of the loud music
ive seen some exhibitionist tzuyu around so wkdkdksks maybe nayeon fingering tzuyu in a dark corner and she takes a while to acknowledge that tzuyu is enjoying it a bit too much and then she starts poiting it out, telling her how much of a dirty girl she is from being fucked in public, asking her to be louder (theres a few ppl passing by the corner and riles tzuyu up even more) and the only thing tzuyu can do is come all over nayeons fingers