The best layover kit is small enough to keep in your personal item and useful enough that you stop resenting the airport.
Pack for control
Bring a charger, battery pack, wired earbuds, refillable bottle, medicine, and one snack you actually like. These are the things airports make expensive or inconvenient when you need them most.
Pack for comfort
A light layer, eye mask, compression socks, lip balm, and a toothbrush can reset your body faster than another coffee.
Pack for leaving
If you might leave the airport, keep ID, payment, transit card, medication, and your onward boarding pass close. Do not bury them in a bag you may store or check.