The convention packing list looks simple until you're standing at your car at 7am wondering where your badge printout is. This list is based on what experienced convention attendees consistently identify as the things they're glad they brought and the things they consistently forget.
The Non-Negotiables
Comfortable, broken-in shoes. You will walk 5-10 miles per day across concrete. This is not a category where fashionable is better than functional. If the shoes aren't already broken in, wear them for two weeks before the convention. New shoes at a convention are a form of self-harm.
Your badge and ID. Print your badge if it's printable. Have the QR code screenshot saved offline in case the venue WiFi is overwhelmed. Bring photo ID regardless — some events require it.
Portable phone charger (10,000mAh minimum). Your phone will die before 3pm without it. Convention halls have crowded charging stations. This is a solved problem with a $25 device.
A bag that doesn't hurt to carry. Whatever you bring will fill up. A bag that puts 8 pounds of convention purchases comfortably on your shoulders for 8 hours is the right choice. Test it before the convention.
The Strongly Recommended
Cash ($60-100 minimum). Some vendors don't take cards; ATM lines inside convention centers are long. Water bottle. Snacks for early and late in the day when food lines are longest. Deodorant in your bag for multi-day events. A light jacket or layers — convention centers are aggressively air-conditioned. Small first aid basics (blister bandages are particularly valuable).
For Cosplayers Specifically
A small repair kit: extra glue, safety pins, double-sided tape, a sewing kit, clear nail polish (for stopping fabric runs), and whatever's specific to your costume's vulnerabilities. A costume that works at home is not guaranteed to work through 8 hours of wearing and thousands of people. Know where your costume's weak points are and bring the means to fix them.
What Most First-Timers Forget
A pre-planned budget (the convention will make you forget you have financial limits without one). Earplugs for evening events in loud spaces. A charger cable that actually fits your phone. Their badge on the second morning after forgetting it in the hotel room on the first afternoon. Sunscreen if any outdoor component is involved. Pain reliever.
What to Leave at Home or in the Car
Heavy items you'll regret carrying by 2pm. Valuables you can't afford to lose in a crowd. Extremely fragile cosplay props that won't survive a full day.