At the end of a long, hot hike, when you arrive at the air-conditioned coffee shop, slip off your backpack, and sit down with an ice-cold lemonade, you might say that this rest stop is just what the doctor ordered.
People say something is just what the doctor ordered when it’s exactly what is needed at the time. It can be almost anything, as long as it's something that brings relief or a pleasant change — like medicine the doctor prescribes when you’re sick. It’s something that makes you sigh, "Aaaaah!"