Are There Improvements that Can Be Easily and Quickly Made Without Buying More Stuff?

I focus too much on bass. :) The other issues besides avoiding room modes are avoiding early reflections, so having enough space to the side and rear walls is important, but so is getting the _right_ toe in and tweeter angle for your speakers.

Audiphiles tend to use laser pointers to point the tweeter right at their ear canals. That’s actually not usually right. Lots of speakers like less toe-in, like Focal or Magico, and many sound better at the mid-range axis or below. B&W for instance.  Often 2-way speakers sound much better below the tweeter axis too.  Only way to know is to listen.

totally. credit card, Crutchfield, Amazon, Safe & Sound

You are correct not to spend any more money with that room. The only free thing I can recommend is bring the chair closer and use a nearfield setup to minimize speakers/room interaction. 

As others have already mentioned, speaker placement and listening position placement.  For my room it was moving my speakers close to positions that were recommended by Vandersteen (intersection of points in the room that are 1/7th or 1/5th or 1/3rd of the width and length of the room) and then moving my listening position from center top of equilateral triangle and back from there.  Soundstage opened up and everything just sounds that much better.  I've also started to learn how to use REW and am able to measure some of the changes to understand the different areas of my room.  Lastly, adding absorption to side wall first reflections was another game changer for me (not free but DIY wasn't that expensive) and also added DIY bass traps to front corners.

listening position height...start low and try higher and higher...