Eh, when I take a shower, I will sing whatever is stuck in my head at the time. Lately that has been Misty Mountains Cold from the Hobbit soundtrack. Freakin' awesome song, but I wish they released a version with all twenty-seven verses, instead of just two.
Anyway, back on track, I also will sing any song from MLP, or any of the Epic Rap Battles of History that I have memorized.
Although I don't get much of this a day, as I refuse to sing in the shower when my mother is in the bathroom as well, which is all but a few minutes of the time that I am taking a shower.