My camera body and lenses are heavy as it is, so when it comes down to what else I pack in my camera bag, I keep it limited and compact.
Remember when preparing your bag, a typical wedding day requires you to be on your feet for at least 8 hours and your body is going to need to bear the weight. I certainly learnt quickly what the necessities were.
Below is a photo of what I usually carry around with me for a shoot or on a wedding day.
Water Bag- This actually clips onto the back of my camera bag so it doesn't take up any space inside. I can't provide a link to this as it was a Montreal travel token but I'm sure you can find these online or at a souvenir shop.
Pelican Cases- I protect all of my cards and batteries with water proof Pelican Cases. SD CASE. BATTERY CASE.
Langly Camera Strap- This is my favourite camera strap. It's made of paracord which is tested to hold up to 550lbs, it doesn't hurt your neck and it can be used as a shoulder strap as well. Find it Here
Tripod Mount- I've included this because this piece is almost always attached to my camera body. Otherwise, it is with my tripod that I keep in the trunk of my car.
Keyring- This is one of my favourite pieces. I clip it to the side of my camera bag and use it to hold all of my keys, however, it has a variety of other uses. Plus... It's locally made :) Available at Wilder Goods
AA Batteries + Charger- Batteries are something I always carry within my Flash bag. The Eneloop brand is great because I go through batteries quickly so these help save me money and their charge lasts a long time in my Speedlights! Available Here
Lens Cleaner- I recommend carrying a lens cleaning kit in your camera bag at all times.
Bluetooth Speaker- I ended up finding this mini Bluetooth speaker on sale for $10.00 CA. For that price tag, I was shocked at how high the volume goes. It has a string to attach to the side of my camera bag. Music has been a wonderful tool for breaking down any ice.
Hair Accessories- Not only are these useful for you, but you'll run into many people that can use a bobby pin throughout the day.
Business Cards- Pass them out!
I hope that you found this helpful :)