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
- FRS
- Amateur Radio
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.
