1. A Compass. This is absolute necessity on highways as detours are almost always un-expected and are always through narrow winding village roads. A compass will help you in these situations to maintain bearings. A GPS may or maynot help, as most of these by-lanes on the highway are uncharted, and mostly un-paved.
2. A Windscreen Wash and Shine. These are available at almost any local auto dealer. And they are helpful. A lot of mud will be thrown on your windscreen from Trucks in front of you and Passing Vehicles. If you have a washer fitted in your car, chances are that will run dry in 200 Kms. Carry extra and refill, specially if you are driving through Dusk or Dawn.
3. One small set of folding table and chairs. Not an absolute must, but if you do want to experience the occasional Alu Puri from a nearby dhaba. This is the best option to have along. Just unfold and you have your own private eatery right next to the highway.
4. Tubeless tires and a puncture repair kit. This is just a matter of choice and peace of mind. Tubeless tires are more resistant to the occasional rogue nail and thorn than the regular Tube Tires. A puncture repair kit allows you to fix that nail easily without running to a repair shop.
5. Water. Carry lots of water when you travel. Either filtered or mineral. Ideal way to carry is in a Plastic Jerrican. A 20 Liter or 2 x 10 Liter is enough for a 1000 Km trip. Water available locally may or may not suit everyone’s city stomachs easily.
6. Get a Local Newspaper. If you are venturing out from one state to the other, get a Local Newspaper of that area as soon as you enter. It gives a good idea of any Political disturbance or Bandh that may foil your plans for the day.
7. Cash and Change for Toll. Any major highway in India is Toll Operated. You will pass at least one Toll Booth in 100 Kms. The Toll value ranges from Rs 30 to Rs 70 one way for a four wheeler. Having change speeds up the process, having cash is mandatory. Calculate about Rs 50 for every 100 Km you travel. Also be prepared to surprise RTO tax collection booths on the highway. This is not necessary for vehicles passing through a state, but it never hurts to be prepared. Tax Value is based on the weight of the vehicle and can vary from Rs 600 – Rs 1200 for a four wheeler.
8. A Map with Highway Numbers clearly marked. It is not necessary that the Map has to be accurate. But it is necessary to have a map with all the road names and numbers marked clearly. Indian Highways are named in Single digits for main roads and three digits for its Bypass or tertiary roads. (Like NH7 and NH207 here). TTK Roadmaps are good to navigate around cities.
9. A Torch. Absolute necessity if you are traveling in the night. The highways do not have any illumination what-so-ever. If anything does go wrong with the vehicle, you will definitely need one to get to the bottom of the problem. Its also a good tool to attract attention in case of trouble.
10. Cash for Gas. Although ATMs and Gas Stations that take credit cards are common in city limits, on highways they are very rare. Maintaining the same brand of fuel is necessary to maintain a better mileage and overall sanity of the engine. Carry some cash extra for those emergency fills. Do not carry too much extra. The chances of theft are very real as well.

Carry a Camera to capture all the moments. Most highways have 80% picture postcard value during ideal weather like Monsoon. Enjoy the drive!