Orkney Islands: See
Prehistoric Orkney: these islands have an amazing collection of well-preserved prehistoric remains. The finest and most extensive are Neolithic or New Stone Age, dating to around 3000 BC. That makes them 5000 years old, older than the Pyramids of Giza, and among the oldest human structures known. They're recognised as World Heritage sites by UNESCO, the pick of them being the Heart of Neolithic Orkney collection around Stenness. There's less from the Iron Age and Pictish Age. Prehistory in this region shades into history sometime in the first millennium AD, when Viking sagas began to describe local places, rulers and battles.
And lots and lots of people naturally want to see them. The standing stones can absorb the crowds and if you just wait aside for 20 min, the tour bus will depart and you can enjoy the lull before the next group. But in the underground chambers it doesn't take many to cause congestion, and at Maeshowe they've had to limit access; it can be booked out for days ahead. There's no need for this because there are so many other high-quality sites, more than a single trip to the Orkneys could encompass, and those off the "circuit" are seldom visited. So spread out and enjoy. Most of them are free to enter, any time of day or night; pre-view them on Historic Environment Scotland website. Most of them just have a signpost and a grassy path leading to a hole in the hillside; the lesser-known just have the hole. If it's been raining you'll get filthy, but you'll really feel like an explorer as you crawl inside - although Lord Carnarvon and Indiana Jones didn't have smartphones to use as flashlights.
The principal sites are:
At Stenness is Maeshowe, Stones of Stenness and Ring of Brodgar, with Skara Brae a few miles north.
Birsay has the Brough (plus later Palace and Skaill House).
The island of Rousay has Midhowe Broch and Cairn, and Yarso.
Less touristy sites near Kirkwall include the Cairns of Wideford Hill and Cuween Hill.
Medieval Orkney: the main structures are St Magnus cathedral and the Earl's and Bishop's Palaces in Kirkwall, and the Earl's Palace in Birsay.
Natural world: Old Man of Hoy, and "The Gloup" at the south-east edge of Mainland, a deep chasm into the sea.