Wipes! - even if toilet trained, bring these for the constant dirty hands & faces
First Aid Kit
Potty - If in-training or recently trained, a potty might be good to bring since some outhouses can be pretty scary.
Variety of toys that you only bring out if it rains (see separate Camping post)
Durable & Washable Toys - (see separate post on Camping)
Snacks - snack mix (see separate post in Food), Popcorn Twists, Fruit, Marshmallows, Meat & Cheese (cut up at home and put in plastic container), etc.
Rug or Mat outside Tent/Camper to keep a minimum of sanding coming inside
Clothesline - (Rope or folding wooden variety) There will be many wet items that will need to be hung up somewhere.
Extra Socks, Extra Blankets