We've been to a number of lovely places in Mexico over the years. We've had warm weather in Puerto Vallarta, Manzanillo, Isla Mujeres, and Cozumel over Christmas. It will be cooler in the highlands -- we were in San Miguel de Allende last winter -- but even so, we swam in hot pools during the day. Mexico City is interesting for art, parks, architecture. Cancun is a little bit canned touristy, with gorgeous beaches. Other beaches can be nicer, but Cancun has a big tourist infrastructure.