The summer is coming to a close, but there's still plenty of time to make sundresses. Especially the ones with the shirred bodices--if you attach two halter straps or four shoulder straps, girls can still wear them with a tshirt underneath in the cooler fall or spring weather, or they can pair them with a turtleneck and tights in the winter (this looks better if you do four straps--a turtleneck with a halter just looks a bit odd).
I'm going to make this one with some Northcott quilting cotton and elastic thread for the shirring. All you have to do is measure from the underarm to the hem for length. I'll write a more detailed tutorial (with pics!) later. You can also use pre-shirred fabric--I've seen some at JoAnn's and if your Walmart still carries fabric, they often have it, too. I prefer to make my own shirred fabric--that way I can make the dress from the fabric of my choice, rather than having to pick from what's been pre-made.
These work for grown-ups, too, if you have the figure to pull it off.