I'll give two categories. First, things you can transport in your car:
Water purification systems
Disease was one of the biggest killers in medieval times. Any one of a number of simple technologies could lower that death count, at least for the wealthy. The best options would be distillation and filtration (tubes packed with sand).
Galvanic cells go back at least as far as the ancient Middle East. However, it is not known what use they might have had beyond possibly shocking people. Basically, ancient gag gifts. They could potentially be used for treating metals, but possibly not under the conditions or with the other materials available in medieval times. I am not sure what other uses medieval society might have for low-quality batteries.
Tin can telephones
Do I need to say more? Additionally, other auditory tricks might be useful, like acoustic mirrors, which could be used to send messages quickly within larger cities/fortifications or possibly detect where enemies are located.
Now, things you can't transport in your car (or might not be physical things):
The idea occasionally popped up of vaccines, but the idea failed to gain traction until after germ theory. If you could sell the idea (it seems like a bad one) to the people of the time, it would go a long way.
Gunpowder is fairly simple to make. Cannons are not that much harder to build, and quite obvious once you have an explosive. Introducing gun powder would completely change medieval, architecture, and politics.
Angled walls (star forts)
While it would not be as large of a help without the above mentioned cannons, angling the walls of castles would help to deflect projectiles. If cannons are not around and you are protecting against catapults, the angles would ideally be only horizontal - as a vertical angle could actually increase the damage done.