The standard 16"x 8"x 8" concrete block found at any hardware or home improvement store is probably the least expensive and longest lasting material. And can be portable, if so desired. Also perfectly acceptable for any sort of raised bed gardening.

But pretty institutional looking if you are going for aesthetics :-))

Sounds good to me. Thank you !

And these or other wall blocks are comparatively stable. Plant things that drape over them to reduce their visual impact.

Depends on how much you want the bed raised.

I have used Bullet-style edger stones (Edgestone is one trade name) two layers high, about 6" total. These will handle round or serpentine layouts, comes in colors.

The concrete blocks mentioned above are fine but they are very heavy and present a razor, abrasive edge. I have topped mine with 2x8 wood. When the cement blocks are placed with the voids side facing up, I press-fit small pieces of wood every second void and attached long pieces of 2x8 wood to them, eliminating that sharp cement edge. This also holds the blocks in perfect alignment long-term.

The ground beneath any multiple-layers of concrete blocks will have to be very compact to prevent uneven settling.