So you want to visit Mexico?
Great! It´s a beautiful country with lots to see and experience, the food is renowned throughout central and South America, and Mexicans are friendly and welcoming. But let´s clear something before we even start, Mexico is a BIG country, and whether you have 3 or 4 weeks, you will not be able to visit the whole country. Here I will give you my idea of a good itinerary to maximise time, while enjoying each place.

The North of Mexico has lots to offer, but it is currently a bit dangerous, and although I loved Baja California (stayed there 3 weeks!), being from the South, I am slightly biased and would recommend saving the North for your 2nd trip to Mexico. (Yes, most people fall in love and come back. My parents even stayed 20 years haha!)

Fligts
I would recommend flying into Mexico City and flying out of Cancun if the price difference to fly out of another city is not too much. This will save you time and money on internal flights.

Internal flights
The luxury bus company ADO is a great option for short to medium distances but some are just worth doing by plane if you can afford it.
For internal flghts, check Interjet and Aeromexico 1st, although these days most prices are similar.

How you distribute your time once in Mexico will be very personal to you of course. But these are my tips as someone who lived in a city and wanted to finish near the sea and ideally come back with a tan.

Let´s assume you have 21 days including 2 days of flights which leaves us with 19 full days to explore and fall in love with Mexico.
I´m going to keep it brief and exclude accomodation and restaurants and just include things worth doing.

Mexico City
2-3 nights
Museo Nacional de Antropologia
Museo Frida Kahlo
EAT.STREET.FOOD.

Mazunte – Pacific Coast (Oaxaca State)
3-4 nights
Surf if you are a surfer, take a surf lesson if not.
Eat ceviche with your feet in the sand.
Visit Zipolite and San Augustinillo

Oaxaca City
1-2 nights
Visit Hierve el Agua, it´s a day trip.
Eat a Tlayuda.
If you have any space in your luggage for souvenirs, this is the place for beautiful and cheap souvenirs.

San Cristobal de las Casas – Chiapas
1-2 nights
Visit en Canyon del Sumidero – day trip
Also a good place to buy souvenirs

Palenque
1 night
Visit the ruins of Palenque.

Cancun
You will most likely land or arrive here by bus. Despite being from Cancun and enjoying visiting my friends I wouldn´t recommend staying over night.

Isla Mujeres
1-3 nights
How long you stay here depends on whether you are happy chilling on the beach. There isn´t much to do on the island but I personally love the slow rhythm of islands and think 2 nights is too little.
I wouldn´t recommend doing the underwater museum. I haven´t done it but I have been told it´s not worth it.

Tulum
3-5 nights
Please note there is Tulum town and Tulum beach. They´re not far from each other and you can either cycle, drive, or get the local colectivos or cheap taxis.
Visir the Coba ruins.
Go to a Cenote. (I love cenote Jaguar! 250 pesos entry)
If you want to see turtles, go to Akumal but expect a crouded beach.
Go into town to eat street food!

If you have more time…
Island of Holbox – min 2 nights as it is far from Cancun. You can get a bus from
Cancun to Holbox and from Holbox to Playa del Carmen, which is near Tulum.
Laguna de Bacalar – 2 nights min as it is quite far from Tulum, but it is beautiful and peaceful. Worth the trip if you can make it.
Cozumel – 1 day, rent a scooter and go around the island.