What to bring

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

• 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)
• Camera: Record proof of reaching the target or take group photos (besides taking pictures of cool stuff)

Also consider:

• Snacks
• Water
• Cell phone (in case of emergencies)
• Games
• Tripod for camera (group photos)
• Book or laptop, if you need something to keep you busy while you're waiting for other geohashers
• Picnic blanket
• Family Radio Service device(s)
• Amateur Radio
• Bug spray
• Rain gear
• Towel
• A Citation might also come in useful.

If you're trying for any kind of MNIMB Geohash:

• A plan - know what you'll do in an emergency, know a way out in case of GPS faliure, 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
• Lights (in case it's winter)
• First aid kit
• Compass
• Another truck if your location is a swamp. Don't say I never told you to.
• A boat.
• Duct TapeStaple Gun