You asked for quick and easy.. didn't say anything about cheap.
Good or Fast/Easy or Cheap pick any two
So .. I'd say pull the interior out (all of the seats, carpet, and down to the door skin) and then apply sound deadening material of your choice.
of course the name brand that gets the most recognition (highly advertised) is:
http://www.dynamat.com/
but there are PLENTY of choices out there such as:
Second Skin Audio was built with one goal in mind: provide the best car sound deadening materials in the world, without exception, and without compromise.
www.secondskinaudio.com
or
https://smile. amazon.com/gp/product/B012B5EMGO/
I personally have used this in my CX-5 to help the
subwoofer (before I upgraded the sound solution).