Here is my list of must haves in the boot of your car. In no particular order.
1. An Umbrella Sure you will not always know when it is going to rain but make sure you keep that umbrella handy. 2. Ice Scraper Well worth keeping all year round. Light weight so these are not going to add much to your fuel consumption. 3. Tyre Pump I use a 12v Electrical pump but you could have a different version. Its great because it has an adapter for my bicycle.
4. Screen Wash
Windscreen Cleaner and Screen Wash. During the winter I use a stronger solution to avoid it freezing on cold days.
5. Meguiar's Ultimate Waterless Wash & Wax
Or similar products are available at most car shops. It helps keep the car clean between washes and only takes around 10 minutes to do the whole vehicle.
6. Spare Tyre or Puncture Repair
Most new cars are not coming with spare tyres now, so if you do use a puncture repair kit make sure you replace it.
7. Hi-Vis Vest You never know if this is needed, you may breakdown and need to be seen by other vehicles when standing at the side of the road. A warning triangle, and first aid kit can be purchased as part of a kit. 8. Torch
Very useful if a problem happens with your vehicle.
This may depend on the distances you drive, but staying fuelled up yourself maybe very important. 10 Clothing I'm not suggesting you bring the whole wardrobe but a light rain jacket/cost perhaps a rug and something to sit on that maybe doubles as a wrap around.
Check to see what countries require certain items to be carried at all times when driving into Europe for example.