Pattern: "Square 78" from 101 Crochet Squares by Jean Leinhauser.
This is the best kind of pattern for busy days. It is so super simple that after the first few rounds, you learn the stitches that create its shape, and you no longer have to refer back to the pattern.
The only problem I had was that the pattern wasn't really explicit about the way the corners should be worked. Normally, you would work your stitches around the chain stitches in the previous round for corners, but for this pattern, you work your stitches into the back loop only of those chain stitches. Other than the last round of single crochet, the entire square is made of double crochet stitches worked into the back loops. The front loops that are left behind when you crochet this way are what creates the ridges between each round. I'm sure you could crochet this same pattern through both loops, that would just change the texture of your resulting square.