Let me make one thing clear before we start with my selection of backgrounds. By 'natural' I mean 'available outdoors'. We are not talking about the backdrops that we buy for the studio. If you want to read more about studio backdrops please click here.
My obvious number one outdoor background is the natural bokeh created by light that's hitting leaves. This is truly amazing and gives me the one-of-a-kind randomness of the environment that I can play around with in Photoshop to produce a really enticing image. All you need is a wide aperture lens (I shoot with a classical 50mm prime) and stay within the range of F2.8 to F4 for maximum sharpness. When I shoot such portraits I keep a fair distance between the model and the background to create a nice bokeh and give my photos the third dimension that most studio shots really lack. You need to know what you're doing here and use a diffused flash, a light reflector or both to make sure that light gets properly distributed on the areas of emphasis.
My second favourite background is the repeating pattern. Such shapes one can not find in nature because nature doesn't work this way but still we can be creative outdoors and find an old fence that we can shoot against. These patterns will make your model's face and body really pop out and you will get the needed emphasis exactly where it should be. The eye will go straight for the model's figure and face. Keep in mind that such photos can be made without a really wide aperture. We can simply use a lens without too much aberrations. Otherwise the straight lines of the background we chose will look curved at the edges of the photo. Another thing that is essential, do not leave harsh or any shadows on the background. If you do your portrait will look like taken with a small compact camera. Get a colorful pattern (a brick wall, a painted or old fence) and you'll find out that the results will turn out to be more than eye-catching.
Autumn leaves present a great opportunity to shoot different angles and experiment a lot. I like going for a late afternoon shoot with one reflector and work until dusk. Such photos give a nice and warm feeling because of the background color and are really easy on the eye.
I wouldn't want to make this list any longer even though I have plenty more favourite backgrounds. I will leave it at 3. Maybe you could post your 3 suggestions and we can discuss them.
Final words, use your background to build up on the idea of a third dimension, mood, season, place etc. Don't just go ahead and shoot like you would at the studio. Outdoor portraiture could be very demanding and usually requires good timing, careful selection of place and even choosing the right model. More on outdoor portrait photography click here.
Have a great day,