The Gobi
Tsagaan Suvarga, the ice in Yolyn Am, the Khongor dunes, the Flaming Cliffs and the ruins at Ongi — the whole southern circuit before you turn north.
Two weeks from the Flaming Cliffs to the far side of Khuvsgul, crossing every landscape Mongolia has without driving the same road twice.

Mongolia is the size of western Europe with almost no one in it, and the distances are the whole problem. A week buys you one region done well. Two weeks buys the desert, the imperial heartland and the northern lakes in a single loop that never backtracks — which is a different trip, not a longer one.
The order matters: the Gobi first while you are fresh for the driving, the Orkhon valley in the middle with a day on horseback and a hot spring, then north into forest and water for the end. By the time you reach Khuvsgul you have crossed from desert to taiga overland, and very few people who do this wish they had gone home earlier.
Tsagaan Suvarga, the ice in Yolyn Am, the Khongor dunes, the Flaming Cliffs and the ruins at Ongi — the whole southern circuit before you turn north.
The waterfall, a full day on horseback among the herding families, and the Tsenkher hot spring at eighty-six degrees to take the driving out of your back.
Khorgo Terkh and the Great White Lake with its volcanic crater, then Khuvsgul — a hundred and thirty-six kilometres of clear water at 1,645 metres.
Uran Togoo's extinct craters, the deer stones at Uushgiin Uvur, and the three thousand rock paintings on Del mountain on the last morning.
The loop needs the northern roads open, which rules out winter. April and May are clear and cold at night; September is the best of it — gold steppe, warm days, and camps that have stopped being full.

Tell us your dates and how long you have. We'll shape a route around them, or point you at the group departure that fits.