- a small spiral log book (as shown)
- an exercise book
- a long strip of paper
or any other clever means of recording a visit.
One cache, Donkey, uses a pack of cards!
The log book is not always accompanied by a writing implement, so best to always have your own. Or better yet, have a signature item to put in each logbook. Some geocachers use a custom or novelty stamp, others use stickers they have printed at home.