The short answer
One day is enough for a taste, not for the city. In a single day you can walk the souks, see the Koutoubia minaret from outside, visit one palace such as the Bahia, and spend the evening on Jemaa el-Fnaa. It will feel fast, and you will leave with a list of things you missed.
In more detail
Treat a single day as a stopover and choose a route rather than a checklist. Start in the souks in the morning while they are quiet, take one palace or the Ben Youssef Madrasa before lunch, rest through the hottest hours, and come back to the square from late afternoon as the food stalls go up.
What a single day can't give you is the slower side of the city: a hammam, a long lunch on a rooftop, a garden in the early light, or a quarter like Bab Doukkala where few visitors go. Those are what make a second and third day worth it.
If you are flying in and out on the same day, a guide is the one thing that saves the most time. The medina lanes are unsigned and even regulars take wrong turns, so a guide who knows the shortcuts turns a rushed day into a full one.
Good to know
- Go into the souks before about 10:00.
- Choose one palace, not three.
- Keep the evening for Jemaa el-Fnaa.
- Download an offline map before you land.