These are items that are always good to bring with you to a LARP, some of them are rather obvious while others might seem a little less intuitive until you run into a situation when you need them.
For All Events:
Boffer weapons (including scabbards and quivers to carry them), Bug repellant, Cash (for food), Costume, Drinks (such as bottled water, juice, or energy drinks, I do not suggest soda though), First aid kit (including any personal medication), Headband (white for out of play, orange for non-combatants), In Play Tools (healing kit for the healing skill, picks and probes for tinkering, instrument for performance), Makeup (if required for character), Non-costume clothing (for before and after game), Paper and Pen/Pencil, Props (for decoration, in-game interaction, rituals, or magic requirement), Physical Representations (such as glow sticks, candies, or vials), Rulebook, Snacks (such as trail mix, granola bars, or prepackaged snacks), Shoes (comfortable, durable shoes are best), Spell packets (for magic users), Sunscreen, Watch, Weapon repair materials (such as tape, foam, etc)
For Two and Three Day Events:
Bathrobe, Blankets, Decorative props (such as candles, standards, or art), Extra costumes, Extra socks, Extra underwear, Fan (for hot weather), Flashlight, Pillows, Sandals/Flip flops (for showers), Tent (for camp-over events only), Toilet Paper, Toiletries, Towel