If you want to have sex outdoors not only because of the closeness of nature, but also because the sense of adventure, then you would probably want to step out of your gates. There is no need to fly or drive for several hours. Even the local forest can be the perfect setting.
There, you might just want to wander in the woods for a short while, until you find a quiet place where you are not likely to be disturbed. There, you can either choose to lie down on the ground, or lean against a tree.
If you opt for the former, then you should always use something to lay on the ground, to avoid getting bit or stung by animals and plants. It does not need to be a tightly woven blanket, but even if you use your coat or shirt, it is better than nothing.
However, if you prefer not to bother with too much setting, then you would probably want to find a tree or bush to lean against. Whichever you choose, it should have solid branches – that are obviously not prickly – and it should be old enough to hold your body weight, but young enough to be flexible for your actions.
Standing positions are the easiest to perform, but depending on the tree itself, you can even choose doggy, but always make sure that the tree is strong and flexible enough for anything that you would do. And as long as you keep your eye wide open, you can enjoy yourselves.