Flat-pack furniture, put together properly
Beds, desks, cribs, wardrobes, and the playset that came in eleven boxes.
The instructions said two hours
They always do. Tall wardrobes and bookcases get anchored to the wall so they can't tip over, which matters more than people think once there are kids in the house.
Outdoor playsets and swing sets take longer. Send us the model name and we'll give you a realistic time, not the one printed on the box.