I love singing in the shower while my smartphone is playing a favorite song.
Ditto!   Though I just usually sing whatever comes to mind.   No smartphone needed. 
I like to play the song without lyrics and sing the vocals part. It's nice because the acoustics and reverberations make it sound like I am a better singer than I am in reality.
Ah yes.. that is a good idea.   The walls definitely help the sound of one's voice.
I have this rack that with tall legs that places two shelves above the toilet tank, which is right next to my shower curtain. I also hang a towel on the shower curtain bar so I can dry my hands then choose a new song on YouTube.
That's perfect!   Just don't go dropping your phone in the toilet and flushing it. 
I was going to say, there have been a couple times I have dropped my phone. Once was on the tub floor which I managed to save and dry my phone. The other time was in my toilet XD! It was hilarious. I was just barely snagging the phone with the tips of my fingers as it kept bouncing around the bowl. I managed to save it from going completely in the water. Ugh.
oh boy.. sounds like quite the adventure!  Maybe get a waterproof case for the next toilet adventure.  :-x 
Toilet Adventure! That should be the name of a cartoon on Adult Swim. It can feature a cartoon version of myself singing in the shower.
Ugh.. Love some cartoon network.  My kids love it too.  So many good shows.