A fan, genuinely
If your ceremony is anywhere warm, indoors with limited air flow, or simply full of nervous energy, a proper fan earns its place in the bag. It is a small, practical thing to have to hand rather than relying on the venue. Plus, a personalised linen fan or a fun slogan will always look good in the photos.
The paperwork
Your marriage certificate application, if it has not already been sorted, plus any rings, vows, and speech notes that need to travel with you rather than the wedding planner or best man. Keep these together in one place, checked the night before, not scattered across three different bags on the morning itself.
The emergency kit
Painkillers, plasters, a stain pen, safety pins, and a spare pair of tights. Nobody wants to need this bag, and almost everyone does.
Pack all of this the night before, hand it to someone else to carry once you are dressed, and you get one genuinely calm hour back on the morning itself. That hour is worth more than it sounds like on paper.