I play random Touhou OSTs and arrangements in my head all the time. I just kind of switch around because I remember a ton of the stuff I listen to.
Also, in band camp today, we got these things called dot cards. They show us where we're supposed to go during our show. On them, there's a symbol(a letter) for what instrument we play, and a label, which is a random number that doesn't go above the number of people playing that instrument. They go together like A3(alto sax), T10(trumpet), F4(flute), that sort of thing. My symbol is C because I play clarinet, and the label happened to be 9. C9. I immediately thought of Cirno. Which is kind of lucky now that I think about it, considering that there are literally 9 clarinets in my band this year.
brb transposing Beloved Tomboyish Daughter for clarinet
EDIT: Wait, one left but it was after the patterns and stuff were made. That really was lucky.