What to bring

It's up to you, of course. But here's what other people have found useful:

1. In General:
   1. GPS / aerial maps / driving directions: Some way to get to your destination (note that GPS won't help you find your way around cities with crazy one-way streets)
   2. Camera: Record proof of reaching the target or take group photos (besides taking pictures of cool stuff)
   3. Some kind of marker that you want to leave on location.
2. Also consider:
   1. Snacks
   2. Water
   3. Cell phone (in case of emergencies)
   4. Games
   5. Tripod for camera (group photos)
   6. Book or laptop, if you need something to keep you busy while you're waiting for other geohashers
   7. Picnic blanket
   8. Family Radio Service device(s)
   9. Amateur Radio
   10. Bug spray
   11. Solid boots
   12. Rain gear
   13. A Citation might come in useful.
   14. PGP Keysigning information.

 42. Towel

1. If you're trying for any kind of MNIMB Geohash:
   1. A plan - know what you'll do in an emergency, know a way out in case of GPS failure, for example know that walking in a particular cardinal direction will take you to a highway, river or road which you can follow to a known location (And know how to determine that direction without said GPS)
   2. Lights (in case it's winter)
   3. First aid kit
   4. Compass
   5. Another truck if your location is a swamp. Don't say I never told you to.
   6. A boat.
   7. A horse|snowshoes|a dogsled team|a snowmobile
   8. Duct TapeStaple Gun