I finally decided on what to make with those left over squares I had lying around. A cushion it is.
I had to make a couple of extra squares to make it big enough (but didn't have enough green for the second round of the last square, so there is an odd one out), and joined them using Carina's method from her great tutorial on joining granny squares. The back is just plain double crochet in the beautiful brown-black that's used in the last round of the squares, but I've already run out of yarn and will have to go buy another ball, which will undoubtedly leave me with another half-ball of yarn lying around (ironic, since this project is supposed to be about using things up).
Once the back is done and joined to the front, and the seemingly endless ends are woven in, I'll felt this a little in the washing machine so it's nice and snug over the pillow form. Can't wait 'til it's done!