Advice on planning a trip to Mexico

A friend is going to Mexico for a wedding over Christmas (Puebla) and me being in Texas at the same time, suggested I fly down and join him for a week (5-7 days). More than likely I'll meet him MC, see the pyramids & a few other sites, then hop on a bus somewhere because we thought it would be cool to spend the new year on a beach, but where is a nice beach close by (yes, I know how large Mexico is)? I once spend spring break in Zihuatanejo, but that was just lounging on the beach, sleeping in hammocks, drinking beer, nothing much more than that.

I don't know the rest of his travel itinerary, but he's there for 3 weeks, so I can imagine it won't make much of a difference for him if we go east or west coast. I'll probably have to find my own way back to MC for my flight back to Austin.

I would suggest checking out some cheap flights somewhere, Mexico City is pretty central and busses are, well, from my experience not a good option in Mexico.
We flew to Acapulco and Cancun from Mexico City.

I took the bus from MC to Zihuatanejo and it was pretty nice. I just assumed Central, South Americans preferred travel by bus and that this infrastructure was better laid out, but that does take awhile. I've never been to Acapulco, but is it pretty touristy? I know Cancun is where all the Americans flee to (Mazatlan and Cozamel as well).

yeah, Acopulco is kind of touristy, are some nice beaches, just looked at photos of my cousin's trip to Peurta Vallarta, and that looked nice.

I personally disliked Cancun. Only monster hotels, no locals live in Cancun. Only advantage is the proximity to the Chichen-Itza pyramids.

Mexico City is definitely worth a 2-3 day visit. Museo de Antropologia, FRIDA KAHLO MUSEUM.
My favorite nighttime hangout place: Plaza Garibaldi with hundreds of Mariachi bands playing.

Closest beach from D.F. is Acapulco, reachable in a few hours by bus or rental car (the toll road is in excellent condition).
Although the beaches in the bay of Acapulco are somewhat disappointing. Great beaches northwest of Acapulco with waves. On that stretch of land between the ocean and the lagoon. The beaches are „owned“ by restaurants, great service cheap prices.

Puerto Vallarta bay can be polluted from the river run off. If PV, stay in Nuevo Vallarta

Manzanillo is a cute little town, with few gigantic hotels.

Best overall beaches in my opinion in Los Cabos (several flights daily from MEX to SJD. Clean warm water, great place if you are into deep sea fishing.
Less humidity, surrounded by desert

Hotel recommendations: Depends on your budget.

Forgot one destination (not a beach town though):
San Miguel de Allende, one of the most picturesque towns in Mexico.