Useful

The GPS, always a must. What would you do without it ?

Spare batteries. Always have some.

A pencil. You know, for small caches that have no room for one. Bring one with you.

Some toys you might trade

A compass. Though not required, it's always useful. And guess what! Works without batteries.

Flashlight. Not only for the evening, but we may have some night caches as well.
Also you may consider having a UV flashlight. Who knows ?

A smartphone can be of use. Search the internet, decode a clue ...

The map. You say it's not necessary? With all those modern gadgets? Are you sure ?

And most importantly : your wit