To stay gorgeous from the ceremony to the final waltz, slip into your make-up bag your morning lipstick, a mattifying powder (your ally against shiny photos), some setting spray and a few hairpins that match your hairstyle.
For sewing emergencies, the essentials are safety pins, a needle with cream or white thread, and double-sided tape.
For your physical comfort, think about discreet plasters (bridal shoes are rarely worn in!), a painkiller, and a few fresh wipes for hot moments. A small, light snack is not a luxury either, as the morning will probably be intense and the drinks reception sometimes a long way off.
For practice, a charged power bank and your service providers' numbers printed on paper: if your phone packs in at 4pm, you'll have everything to hand.
This kit fits into a make-up bag. It weighs nothing, but is worth its weight in gold.