Your listed conditions just eliminated all the best, mulches. You could purchase knee pads and use mulches of almost any type which will control weeds, provide some nutrients and improve the soil. If you don't choose this, you are left with plastics, yuck!!

well, I'm confused;
it sounds to me like the only thing ruled out was grass...

Put down several layers of newspaper or cardboard, overlapping generously, & cover with sawdust, hay, and/or pine needles.

Have fun!