What I missed most on recent trips

It's going to be with me for a while

I’ve traveled a bunch in September—to London and Vermont and not to toot my own horn, but I’m a pretty good packer. I mostly wear black which makes it easy to mix and match and I have travel sized toiletries, the perfect bag, and a beat up Away carryon from 2017 that I’ve packed and unpacked countless times. I usually bring a slim Muji notebook with me on the road but on my most recent trips, I’ve left one thing behind and really missed it. I got it for my birthday (thanks, Leah) and will be making suitcase space for it on longer trips going forward.

What is this habit forming item?