I have made it a habit to carry a "help bag". Its full of things that can come in handy during a wedding, but not just for the Lead photographer.
Aspirin, double sided tape, scissors, sewing kit, allergy meds, bandaids, screw driver, nail-file, collapsable shot glass, sticky puddy, eye drops, flashlight, swiss army knife, cleaning cloth, chapstick, glue, gaffers tape, (the list goes on)
Nearly everything has come in handy at one time or another.
AA batteries, as well as camera batteries. At least enough to power all transmitters and receivers, as well as any strobes that require them. At least one extra camera battery for the Lead. Having a spare flash around, in case bulbs break, get dropped, etc.
Extra Memory- A few cards, one at least for every type of camera being used.
You should not have any. Formals for instance there is very little need to shoot right next to the Lead photographer. Besides it can end up confusing subjects on who to be looking at. If its not already set up, reception lighting when needed can be set up during this time. Breaking down other gear, or moving it to your next location. Candids of people not getting formals taken. Details of reception hall, cake, place settings, guest table.
I like to think of the lead photographers gear as my own. If there is a chance something can get knocked over fix it. If something gets left behind pick it up,
Things that will make a mark
Snack, it never hurts to have a power bar, or nature valley bar around. While most of the time these days we are taken care of for dinner. I can't tell you how many weddings start around breakfast with little to no break or access to food until dinner.