One Important Accessory I always have and would also suggest others to have are TOOLS! Yes, have seen people with nice rich cars stranded on the highways looking out for help from other cars, say for a torch or a spanner or a tow rope many to name.
Many people out think that their car is highly sophisticated and will never break down, atleast a majority of them think and that is wrong.
A few tools / accessories which I consider a must is :
* A good Compact bright Torch.
* A good capacity tow rope, It can atleast help a family stranded on the highway.
* A swiss knife
* A basic tool kit, tubeless puncture kit, car air pressure pump, pressure gauge.
* Latex can hold hose leaks atleast fuel line leaks, so probably a couple of condom packs (This may look offensive but its the fact that it helped me once with a fuel pipe leak on my old MPFI Maruti 800)
* Car Chargers
* Umbrellas, Bags, Plastic bags (specially for passengers who vomit on ghat sections, You dont need to worry about the window weather strip and doors getting spoilt)
* Defogging spray if your car doesnt have a rear defogger.
* Microfiber cloth, Josapu duster
* Lastly on a long drive its better to keep a pack of biscuits and a bottle of water.