Best Route Shape
A strong first Mexico route starts with Mexico City, adds Cancun or the Riviera Maya for beach time, and uses Oaxaca if food and culture matter more than another resort day. Domestic flights make the route easier than long overland transfers.
Mexico City First
Mexico City deserves at least three nights. Use neighborhoods, museums, markets, parks, food, and a day trip to ancient sites as the backbone. Choose accommodation carefully and avoid treating the city as only an airport stop.
Beach Extension
Cancun and the Riviera Maya are the simplest beach answer because flights, resorts, tours, cenotes, and ruins are easy to organize. The tradeoff is that some areas feel more international resort than everyday Mexico.
Why Add Oaxaca
Oaxaca gives the trip food, markets, mezcal, crafts, festivals, and a slower cultural layer. Add it if you want Mexico to feel deeper than city plus beach, but protect the schedule from too many short flights.