Bongo Fury

Flip flops for showering, easier than shoes/boots
Pegless washing line or some ultra compact means of hanging wet towels n waterproofs.
Warm socks in case anybody gets cold feet

As Alison says...battery or solar colour changing lights...brilliant for the kids (and adults). Also good for finding the van/tent at night. Years ago when kids were small we took a couple of cheapo walkie talkies to keep tabs on 'em.