In ROTPL megaliths, stone circles and standing stones created centuries ago by both giant and druid, are imbued with the power inherent in the land. Here I'll talk about some of the ways standing stones can be used in your game.
Giant stones
Back when the land was new the Worldspeakers used their new-found magic to craft impossibly huge standing stones carved with words of power. Generally these were to denote territory and ward off anyone who might attack the tribe, but others were made to heal after battle. In some cases stones are situated near baths crafted by Giant masons where warriors would lick their wounds in the presence of healing magic. Rugged giant runes are carved into every surface, though by now the magic within is latent at best. Reading these runes requires both an understanding of the Giant language and a severe History or Languages (Mind) check (runes are generally weathered away and really hard to read). Usually it's easier to get a Giant to read it for you, but that always comes with a bargain. Speaking the words activates the magic. Healing Stones usually will heal everyone standing in the circle 3d8 once per sunset. A cunning folk can spend 3 spell points to gain an extra d8 healing for the group by magnifying the magic through themselves (often leaving them a little frazzled).
Druid stones
Stones erected on the behest of the druids are much more recent than the Giant stones and serve different purposes, mainly to venerate the pantheon of the Perilous Land or to mark a magical treasure. These stone circles are much smaller, from 10ft stone slabs to 1ft rocks compared to the behemoth 20ft or more of the giants. Like the giants the druids carved intricate runes into the stone for various purposes. These runes are in the esoteric Druid language which requires a tough Language (Mind) check to read. The magic within is younger and therefore much more potent. Some singular stones have the symbols of the gods etched into them, usually coupled with offerings of mead, flowers and food depending on the god. Druids gather around major stone circles like the Ring of Drydych in Ascalon on feast days to perform ceremonies (coming of age, crowning).
Doom stones
So called because many were created in the Age of Doom, Doom Stones were generally created by unfortunate souls being turned into stone, or evil creatures becoming stone after being defeated by the forces of good. Long Meg and her Daughters are Doom stones in Hutton, created when a family of witches was defeated by Merlin himself. While Doom Stones aren't magical (even if they're a result of magic), they can be reversed through powerful incantations and the right item. It's usually thought of as bad luck to camp near a set of Doom Stones. Anyone who does suffers a Setback on a single check determined by the GM until the next sunset.
Secondary effects of standing stones
Standing stones attract various denizens of the Perilous Land to them. The decaying Worldspeaker magic can only be felt by those of Otherworldly origin, so it's common to find fairies, bugbears, gnomes, bogies and pixies in and around Giant stones. Druid stones attract those who wish to pay their respect to the gods, as well as spirits who are accidentally tied to this world by the stones. Doom stones often attract black dogs, hags and werewolves. Witches and crones tend to stay well away from Doom Stones.
Quest ideas using standing stones
1. The bones of the Worldspeaker Vardan are believed to be buried near a Giant stone. They are said to contain properties to reverse death. When a knight falls gravely ill, the heroes must find and gather the bones on sacred giant ground.
2. Every Yule a local village is terrorised by phantom riders tethered by the local druid stone.
3. A group of Doom Stones suddenly turn back into the villagers they once were. When integrating back into society others notice they've returned... different.
4. A recently reactivated Giant Stone has caused a rift to open to Otherworld. A mysterious Piper is enchanting the local children and leading them through the rift.
5. Arthur wants to take a Giant Stone as a tactical position to use its healing properties. Mordred has the same idea.
6. A spirit appears at a Druid Stone pleading to be reunited with his love who is still alive. The problem is that they disappeared into the wilderness a year ago in their grief.
7. Pixies are hosting an annual Pixie Ball in a Giant stone circle. The heroes must magically disguise themselves as pixies and attend the ball and get hold of a pixie flute.
8. A thick dogs has rolled over a town. People are being taken by a group of hags, attracted to the Doom stone that a local cunning folk has taken into their home in order to reverse the spell.