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
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