Banff and the Canadian Rockies
Turquoise lakes and a mountain highway that's the point of the trip.
Waterfalls, black sand and geothermal water, all within a day's drive of the airport.
Iceland's appeal is density of landscape: within two hours of Reykjavík you can stand behind a waterfall, walk a black-sand beach and sit in a hot spring. You don't need two weeks to see something extraordinary, which is why it works so well as a long weekend.
The two hard constraints are weather and daylight. In December you get four usable hours of light; in June you get twenty-two. Both are great trips — they're just completely different trips, and the plan has to match.
Summer gives you endless light and open highland roads. Late winter gives you dark skies with far better weather than December.
A workable shape rather than a schedule to obey — two fixed things a day, and room to change your mind.
Rent a car — it's not optional outside Reykjavík. Take the gravel and wind insurance, check road.is every morning, and don't attempt F-roads without a proper 4×4. Fuel and food are best bought at Bónus or Krónan.
Iceland is the trip where splitting the car cost changes everything: four people in one rental turns a very expensive destination into a reasonable one. Sort the driver rotation and the fuel-splitting rules before you land.