The only temples in Germany were dark and ancient groves, consecrated by the reverence of succeeding generations. Their secret gloom, the imagined residence of an invisible power, by presenting no distinct object of fear or worship, impressed the mind with a still deeper sense of religious horror; and the priests, rude and illiterate as they were, had been taught by experience the use of every artifice that could preserve and fortify impressions so well suited to their own interest.Anyone studying late Roman and Byzantine history has to come in contact with Edward Gibbon's epic History of the Decline and Fall of the Roman Empire, whether or not you actually agree with its basic premises. Despite some faults in his analysis, Gibbon was generally a remarkable writer and created some vivid pictures.
Gibbon's picture of the Germanic religion draws mostly on Tacitus, who of course was writing not as a disinterested observer but as a partisan for Rome. But the description strikes me as excellent fodder for a type of hexcrawl location. This was doubly confirmed when the picture I found when poking around for a good illustration was one of St. Boniface – the "Apostle of the Germans" – hacking down a tree dedicated to Donar or Thor (actually recorded as "Jove" but theorized as one or the other according to the interpretatio romana).
I generally see pagan religions as sitting outside of the cosmic conflict of law and chaos. The powers worshipped are ancient but are deeply of this earth. This fits squarely into the picture that we see of these "invisible powers" that were the residents of the Germanic groves. Gods of this world, as opposed to the cosmic deities, are similar to the chthonic deities of ancient Greek paganism, earthly deities, frequently associated with snakes and the underworld.
D&D does have one type of forest spirit associated with it, in the dryad. This is quite possibly going to lead to a PC being Charmed and led off from the party, never to return from the forest. I have rarely or never seen referees using this sort of encounter, but to me it strikes a tone of mystery and danger that is something a bit more than just damage. And it is fitting that dryads sit solely in the Neutral column of OD&D's alignment chart.
None of this is to say that forest spirits can't possess the threat of physical violence, but I really love the theme of losing a PC not just physically but spiritually to the forest. It's a fascinating way to make PC loss much more than something easily remedied by a Raise Dead spell (though other spells such as Wish may suffice). Perhaps the central tree of a sacred grove has such powers, and can ensorcel unwitting PCs – unless a Lawful cleric comes along and plays the role of St. Boniface.
What I find most important in this location is the way that they instantly convey a sense of dread by virtue of lacking a central cultic focus. Nature religion was generally not a hippie celebration of the earth, but a fearful and tentative glimpse of the supernatural. We see elements of this in the strange vision scenes of the History series Vikings, where characters often have powerful, even terrifying glimpses of the future. This kind of atmosphere should be fair game if PCs spend time in the groves, including weird and possibly unreal encounters with animals laden with symbolism and the like.
The fear that Gibbon saw at the core of ancient paganism is something that so often feels lacking in D&D's take on religion. These locations are excellent ways to change that.