A Sweater
At home, we get to set the temperature. This is not the case in most other places, especially in public transit settings. To make sure I’m comfortable no matter what, I bring a sweater: my go-to is a blue wool zip-up cardigan I’ve had for about a decade now. Over that time it’s kept me warm on planes and hikes, and severed as a pillow on more than one occasion in warmer weather. While it might take up some room in a bag, this is a must.