Newgrange in Ireland, once known as
Brugh na Boine, has a neolithic stone tomb that's designed so that the light at Winter solstice enters the tomb and shines on the wall.
It's an important site archaeologically, artistically and mythologically.
It's also just plain stunning.
Solstice is a big deal there.
It's on my bucket list to go there, especially at the solstice.
My family is from Ireland (Kerry & Mayo) so I used to go back often and have visited Knowth and Newgrange several times. The last time I went Downth was only starting to be excavated. There are a LOT of other ruins/passage graves/ artifacts in that area - many are not excavated because people there consider them sacred.
Ireland is filled with old ruins, many dating back as far as the paleolithic era, many untouched, some strange enough that no one knows who built them. In Co. Donegal for example, there are a number of passage graves with the stones arranged in the shape of a boat (these predate the Vikings by several thousand years). The graves are just sitting around in fields and you can just walk out to see them. Stone circles are plentiful, as are standing stones marked with ogham characters (based on the Greek alphabet I believe).
If you'd care to venture north to Ulster, there is
Emain Macha (which I was told appeared on some ancient Egyptian maps), it was the seat of the bronze age Irish kings and where the famous knight
Cuchulainn (the hound of Culan... real name Setanta) lived and died.
Ireland has a history filled with incredible legends that go back 8,000 years. The people are wonderful, and in the small towns especially, nights in the pubs are fantastic.
ETA: If you want to see the winter solstice at New Grange, well good luck! It's booked up decades in advance... but they have a light show set up in the grave so you can see a similar effect.