Problem: the girl doesn't know the ideal swimming hole for her to skinny dip in exist just a bit out in the woods behind her backyard. Well, nobody does, because it was just developed by a guy who wants to watch her do just that. How he can get her there without (before) her learning about his existence?
[Creepy alien ramblings ahead]
Johnny, a guy (could be any age, but most probably) about 16, spending the summer at his grandfather's (ex-)farm he calls home despite actually attending quite prestigious inner city school at winter time.
For almost a mile grandfather's lands border on a smallish river (in typical crossection fifty-twenty yards of crotch deep water, but there's rapids, islands, slow wide sections, ten feet deep holes, etc, average drop of 1.5 meters per thousand (with is considered rapid river for the mostly flat area)) and the house is right on the upper bank, inside a wide bend.
There's a bridge over the river three-four hundred yards away, and by the bridge there's a local mini-beach with the central semi-public unregulated swimming hole. It's too crowded for reliable skinny dipping (average visitor density around one family or other group every hour to half hour during hot weather, perhaps up to hundred unique visitors per season) but there's enough stray attempts at that and careless clothes changing on-place to generate peeping interest by our protagonist. Who doesn't peruse the public place himself at all, even if do some volunteering maintenance work there occasionally.
They have a private place right next to the house where he as rule, his father when home for weekend, his older sister lately, and even occasionally his mother swim naked. But NOT in every possible combination together, those rules are complex, confusing and crumbling, at least at part by nudges of our protagonist who takes credit for training his sister into (very limited) nudism, although that might be delusional arrogance. And yes, he's perving on her secretly sometimes even though she says she's feeling better being nude in his presence than alone. Said grandfather is still around the house too, but he doesn't swim in the river, too old, old enough to claim swimsuits were invented by communists just to screw up the world order, while he also considers nudity insulting to Work or Food, so open full time nudism is out of the question for that reason alone.
So, someday this guy spies a new girl at the bridge swimming hole, Kristine. Apparently she's a city girl deported for the summer to the aunt's house. He gets reasons to believe she would skinny dip given fair chance, or rather, that either her own aunt or her new girl friends, sisters Rebecca and Suzanne from across the river, would incite her. All they need is a bit more privacy and feel of security.
He stalks her and discover she's actually next door neighbor, less than half a mile away the other direction.
So she actually have to travel all around his lands, over a mile by road, to get to the public swimming hole, while she might go two hundred yards through woods to find a very secluded place on the river, was there any such. Actually there is, and quite obvious for someone like him who knows every stone on the riverbed and every three in the forest. So he develop a perfect swimming hole for her (working modtly in the early morning hours), of course, perfect for him, in a way he could half-effortlessly observe it. He makes a trail through the woods to her backyard. All of it looking perfectly natural and incidental. Rebecca and Suzanne could use that place too, even coming in from the other side, not as close or effortlessly, but with obvious benefits.
The missing bit is, how to make Kristine (or Rebecca, probably for the same end results) to discover the place, without raising suspicion to himself, preferably without Kristine learning about his existence before she's comfortable walking naked through his woods.
~~~
I have some ideas... but would like to hear what people might find more believe, the setting is alien enough for potential readers I'm afraid. Then, almost anything short of direct mind control might work.
1) The natural, "historically correct" approach would be just wait. The girls could just stumble upon it, presumably they are on a lookout for a more private venue. But that's random chance, with may never happen, especially if the setting is today-ish instead of nineties when teens were still a bit more bored and thus curious about surroundings.
2) I don't know yet how involved I want Rebecca in this... but she's a witchcraft apprentice of her grandmother, who has a nearly century old thing with the grandfather... but that's all probably a different story and it's not immediately obvious how to leverage in this. Other than how it happened, but that was the other way around, maybe (I can't know), and would merge stories that might not want it.
3) there might be way to steal Kristine's smartphone while she's working naked in the greenhouse (yes, she's observed doing that at some point), but again what's the point? Would she become curious enough about nature pictures on her phone she doesn't remember taking? Would even notice? And we want her to feel secure, not haunted.
4) the guy could get his cousin's dancing braids (or get his own, anyhow) and impersonate a "white rabbit" naked "girl" to be followed in quick flashes at distance and dissapear
5) he could plant the idea with said aunt of hers somehow, but while she's got a creepy bone, that would make her more alerted than not, and I don't think full on collusion out of the blue is a way for them, even if that goes with obvious benefits...