I love stone circles, as found in the British Isles and the Atlantic seaboard of Europe. I generally use them as centers of worship for elves and druids. I have also used them as magical gates to the faerie realms. So here's a table for people to use.
Stone Circle Encounters
1. A small group of elven archers maintains the circle. They have a permanent camp set up in a nearby copse of trees. They generally won't disturb people who are there for worship or just to rest while hiking, but will attack any evil races who attempt to approach the stones.
2. A magical gateway opens as the group approaches. On the other side they can see a great forest. No one is around.
3. A druid maintains the circle. He has a small hut nearby. The locals often deliver fruits and vegetables, in return for his blessings.
4. A group of goblins is defacing the circle. They are attempting to push over the stones.
5. Pixies and brownies live around the circle. They will play pranks on any who approach, and may steal a small item.
6. A necromancer has perverted the circle, using it to raise undead. He places corpses within the stones and casts his spells. All undead raised here have maximum hit points and gain +1 to their saving throws.
7. The circle is old and abandoned. Weeds and brush grow throughout the area. Some stones are on their sides. Perhaps with care, it may return to its former power.
8. A cleric has built a small manor near the circle. He considers the place to be a haven of sin. Some pixies have lived here for ages, and torment the cleric daily.
9. A druid lives in the circle, under the open sky. He has only a bedroll. The local fae respect him, but the human baron who controls the land fears his power. The druid is unhappy with the baron, who has started logging the forest.
10. The stones have glowing blue runes marked upon them. During thunderstorms, the entire hilltop shines and can be seen for miles around.
11. The stone circle marks the resting place of a great elven lord, who fell in battle against an evil god. Elven sentries will attack any non-elf who approaches.
12. The stones are pitch black and radiate evil. Monsters are attracted to the area and are encountered very frequently.