The best delay kit is boring, compact, and always in the bag under your seat. It does not need to solve every problem. It just needs to keep you comfortable if your checked bag or planned dinner is suddenly out of reach.
The core kit
Pack medication, chargers, a small toiletry set, a clean shirt, headphones, snacks, and any document you would hate to lose. Keep it simple enough that it stays packed between trips.
Comfort extras
A light layer, eye mask, and refillable water bottle can make a long gate delay much easier. Skip anything bulky that makes the bag annoying to carry.
Why it matters
Delays are easier to handle when your basic needs are not trapped in an overhead bin or checked bag.