A pack of 10 of those little stubby Geocaching pencils. They are about 8cm long, so they are even ideal for smaller cache containers. It's best to leave a pencil in your cache, as pens don't work forever !
Perfect size to leave in my cache and also to carry as extras in my kit to leave in caches that are lacking them. Customer service was first rate! Shipping was speedy to my country town location and my purchases were well packaged