What's the best way to spend $100/€90/£70 on your car?

A full tank of gas.

One new tyre #onetyrefire

3 laps on the nurburgring!

Wax and orbital buffer

A weighted knob. Or if you have one already, some bushings somewhere appropriate.

Buy a car

Getting a full professional interior cleaning

Petrol

Clean it

Fuel for roadtrip