Creating a druid stonehenge puzzle

My group of players (3rd level rogue, warlock/monk, paladin/fighter, ranger, sorcerer, druid) have stumbled upon a stonehenge in the woods. This coincides with the introduction of a new druid PC. They're overdue for finding a significant treasure haul, and this seemed a nice place to insert some treasure - particularly spell scrolls for the many casters in the party, since that seems thematically appropriate to druids. I also plan on reusing the stonehenge later as a recurring element.

I'd like to create a Druid-themed puzzle for them which allows them to access some kind of spell scrolls (or maybe bonus known spells).

I was inspired by this puzzle: http://puzzling.stackexchange.com/q...-a-five-pointed-star-using-two-straight-lines which maps well to the stonehenge map we're using (see attached image).

I know there's a Druidic inscription around the edge of the stone dais that will have a riddle.

I'm sitting down to work out the specifics, and would love any ideas folks have!

For example... taking this...

This puzzle consists of counting ten triangles (check three sides for each one, remember that there aren't exist triangles with more than three sides) using two straight lines that cross the figure wherever you want.

Click to expand...

and turning it into Druid-sounding prose like this... (work-in-progress)

Two rays, of sun and moon, joined at dusk, apart at noon,
Shine upon this grove and make the blood-marked stone take
Hold of the wheel of life, distilling wisdom from the strife.

Stone to stone shine the rays, o’er star of the Old Ways,
So the pillar from the circle take, strumming trigons in the wake
Till from the star are born nine siblings on the path of thorn.

This seems like a very difficult challenge. It's a non-trivial math problem nested in a riddle intertwined with metaphors.

Test your riddles beforehand. They are very hard to make challenging while not impossible.

I rarely require more than two puzzle clues to solve the larger puzzle. I want the PC's to find over 80% of such clues. As there are two clues, they should solve the combined puzzle about 70% of the time if that is part of the campaign arc. Players feel clever for putting the two items together... perhaps disproportionate to the difficulty of the challenge.

Perhaps a wood carving of druidic activity raising hands to a smokey offering is nested in a prior adventure. The specific moon should be given to inquisitive PC's... a full moon by many druidic traditions. Clever PC's can arrive at the standing stones in time for the correct moon. The runes on the altar could poetically invite the faithful to burn mistletoe in offering.

"Be ye strong of heart,
A flame of mistle twig you shall start."

Slightly archaic, but enough to prompt most PC's.

(Traipsing into the woods looking for mistletoe.) Under the correct moon, the spell's formulation appears in the smoke. Do different moons or offerings produce different spells?

If you go all PBS-Nova-Astronomy on this puzzle, you will be faced with either abstracting your fantasy astronomy into a bland knowledge check or invested too much effort into building a working physical system.

I don't understand how your poetry relates to the picture at all. It's all so dense.

My players stumbled over a single sentence last week because I accidentally split it up to seem like two separate phrases. And I thought it was so very simple.

I have come to believe that the best puzzles in this game are simple ones that require the barest amount of thought to solve, but are placed in ways to make it interesting. Give them a sentence that tells them secrets are unlocked by an offering of mistletoe, then have the treasure cache revealed when they burn some mistletoe on the altar (bonus treasure for burning mistletoe picked during a full moon)