This is something we've never tried but always think about. Hanging curtains outside seems a very "vacation" detail so we can imagine it would make for quite an escape in our own backyard. These are just a few examples of outdoor curtains that inspire us to do something similar at home:
As for what material to use outdoors, we love the plain white sheers as shown in the first image above. For something light and airy, we might look to a parachute fabric. For a heavier option that might provide more shade and comes in lots of fun prints, there are outdoor canvases made of polyester so they can be wiped down with a damp cloth.
We love the example in the second image, which is an unbelievable DIY. Check out many more pics of it and a project description at Knight Moves. And see Heather Bullard for one more awesome outdoor curtain DIY!
Any other suggestions for curtain material for use outdoors?