New Hampshire does not require adults aged 18 and up to wear seat belts. Punishments for minors without seat-belts apply to the driver, and are 25 dollars for the first offense, and 50 dollars for the second or subsequent offenses. New Hampshire is the only state in the USA that does not have any seat-belt laws for adults.
Worth noting, enforcement of this by law enforcement agencies is only done as a secondary action when a driver of a motor vehicle has been cited or charged with a violation or some other offense. Make sure your children buckle up, and you can feel free to take your seat-belt off, even if you're in the front-seat or driving.
Pickup Truck Beds
New Hampshire also has no laws prohibiting passengers from riding in the back of a pickup truck.
You may be interested to hear about New Hampshire Helmet Laws.