Recommendation for NPCing:
Black clothes you can be mobile in. A chunk of your time is almost certainly going to be spent in combat, and this helps you.
Good footwear you can run in that can handle varied terrain. If they're waterproof, that's a plus. Lightweight hiking boots/combat boots are fairly common.
Several pairs of socks, -especially- if your footwear isn't waterproof. And if you manage to snag a pair of waterproof socks (I have a pair of neoprene ones, myself), this is even better. And by several, I mean that nobody would look down on you if you brought eight pairs of socks.
Sleeping mat: Most of the bunks do not have a mattress at our standard Oregon campsite. I've always been a fan of the $20 foam mat you can get at Fred Meyers or Home Depot, personally, but whatever you think will work for you. Additionally, you'll want your own pillow and sleeping blankets/bag/etc.
Toiletries: We have bathrooms and heated showers, so if you like to stay clean (I know I do), bring what you need for that.
Optional: If you have your own costuming you'd like to bring, feel free, though we have and provide makeup and costuming in Monster Camp. I recommend avoiding anything distinctive that you plan on using for your PC, as your PC won't be present.