As a vocalist, I'm always singing in the shower. Sometimes idly with songs that are just stuck in my head, and sometimes on purpose. The humidity and warm air is actually good for relaxing and warming up the vocal cords.
If I have to be up early for a rehearsal or I have to sing early, I start with deep breaths in the shower to get some slight vibration going through the muscle, and then I move on to gentle warm-ups. Usually ascending scales first, and then descending scales, and then some vocal agility exercises. It's easier to hear the accuracy of melismas, riffs and runs when singing in the shower. Sometimes I work on extending into my whistle register, but I usually don't do that until later in the day simply because it's a workout for the vocal cords, and not really something that should be done without having fully warmed up the voice.
If I'm not actively working on vocal technique, I'm usually singing along to whatever song that comes up on whatever playlist I put on. There's usually a lot of 80s rock, 90s boybands, and Beyonce, haha.