Zinnias like sun, bloom from Spring to Fall, and come in a wide range of colors, flower forms, and plant habits. They grow fast and are easy to grow.

ZM

I forgot to mention -- it's pretty small. An enclosed square, about 6' x 6'.

Smaller zinnias. Zaharas or Profusions.

Zinnias are always a good choice! And there are zinnias in every size from 6 in to several feet. But aside from zinnias, I tend to think in terms of what else will grow and be happy in the heat and the sun. For me, that includes annual vinca (though perhaps these may seem plain jane to some people) and johnny jump up (viola), which grow easily from seed and I think are really pretty. Viola are fairly short, only about 6 or 10 inches.

Marigolds do well in the sun and heat, though they aren't my favorite. Petunias don't like the heat as much. They can't hardly handle July in my zone, so I don't know what they would look like for you in August. You might also consider annual salvia.

How about the design of the plot? For example, tall in the back, mid-height or clumping in the middle, and low or a little vine-y in the front (such as the wave petunias). However, a caution about the petunias is that after they bloom the first time, they look a bit scraggly unless you deadhead or trim the stems back a bit.

Being that it is 6x6, maybe there isn't much space for a bush or shrub, but if you decide for one, then there is definitely an opportunity for having some nice-looking plants there year-round.