So not bathroom related tasks, but more like some arbitrary thing you must and always do daily.
For me, I watch Anton Petrov’s daily white paper summary with dinner since some time in 2018. Even when New Pipe is down, I hit up Vimeo or Odyssey to watch Anton.
I think there might be an objectively true answer for most (all?) here: wake up.
There might be some all nighters, but besides that, every day for almost everyone starts with waking up, which is also arguably an important part of the daily routine.