I don't know if that's completely random, whether it's something specific about me, whether it's because I will spend longer in a bath than a shower, or because I enjoy having a bath more, and tend to relax more. Or some other reason.
The key is that the activity has to be both relaxing and routine (double "r"...). For you, it's bathing, for others it's showering, yet for others it's walking in the woods. If the activity is not relaxing, you will be preoccupied with how to control it and with resulting stress. If the activity is not routine, you will be focused on its novelty rather than letting your mind wander on its own.
It's just that in a shower I never decide to relax and enjoy it, it's always a method of cleaning, or of waking up, or of heating up, not an activity for the sake of enjoyment.
My best idea generator is the walk away from my desk & to my car at the end of the day. I have solved so many bugs immediately after exiting the office building...