Lip balm, cotton swabs, makeup remover wipes
If you try and do your makeup at the venue in a frantic rush, more often then not it will end in disaster. And there’s no time to completely re-do your entire makeup either! But if you do need to remove a bit of makeup and re-do a small area, a bit of lip balm on a cotton swab will do the trick.

Also, making the trip home in full cosplay makeup can be a bit awkward sometimes. So if you want to avoid all that, just keep a pack of makeup remover wipes on hand and scrub down before heading home.

Drinks
Heatstroke from dehydration is an important one to watch out for, especially in summer! There are times when you can’t leave a venue in full cosplay attire to buy a drink from the convenience store or vending machine, so it’s best to buy one before going in. (Rules and designated eating/drinking spaces differ at each event, so you should ask the staff for exact details.)

Essentials Bag
There’s so much stuff to take that people tend to want to throw everything into one bag, but you’ll want to have your camera, a drink, and your valuables with you inside, so it’s a whole lot easier if you leave your main bag with your change of clothes in it at the baggage holding service and keep a smaller bag on you with your essentials in it throughout the day.

Safety pins, double-sided tape, quick-drying glue, etc.
Your outfit and props that were in perfect form when you left the house can start to come apart amidst all the frivolities at an event. With these items on hand you can make emergency repairs in next to no time at all.

Business Cards
Cosplayers will often trade business cards with each other at these events, listing their names (cos-names), social media accounts, and links to sites where you can view their cosplay photos. It isn’t rude to not have a business card, but it does make for a good icebreaker.

Hat
If you’re going to wear a wig for an extended period of time, you’re going to get wig-hair! (The wig version of hat-hair.)
Once you take off your wig the hair underneath is going to be flat or sticking out and just looking all kinds of ridiculous. Sure you could try and fix it…or you could just take a hat and save yourself a world of trouble.