For $6-10 U.S., you can take a roughly 1 hr bus ride from Mexico City to Cuernavaca, the summer home of emporer Maximilian. The busses are the nicest I've ever seen, with "in-flight" movies (English with Spanish subtitles, as often as not), a stewardess, and snacks.

Also called "The City of Eternal Spring," Cuernavaca is spring-like almost year-round. In addition to the vacation homes of the Mexico city elite, it has many immersion schools for those who want to learn Spanish. You typically go to class roughly half the day, and board with a local family (room + board in 2003 = $17/day/person) with whom you can practice what you've learned. Our family went to institute Chac-mool and had a terrific time. (See their website, and others, for details)

Cuernavaca is also a relatively short trip to many archeological sites, and to places like Mexico City and Taxco (a silver mining town).

Acapulco
Agua Prieta
Aguascalientes
Ajijic
Anahuac
Apizaco
Atizapan de Zaragoza
Bahia De Kino
Bahias de huatulco
Baja California
Barra De Navidad
Basconcobe
Benito Jurez
Boca del Rio
Cabo San Lucas
Campeche
Cancun
Chetumal
Chiautempan
Chichen Itza
Chihuahua
Ciudad juarez
Coatzacoalcos
Colima
Copper canyon
Cozumel
Cuatla
Cuernavaca
Culiacan
Dolores Hidalgo
Ecatepec de Morelos
Ensenada
Guadalajara
Guanajuato
Guaymas
Guerrero negro
Hermosillo
Irapuato
Isla Mujeres
Ixtapa-Zihuatanejo
Jalapa
Jerez de Garcia
Jerez Zacatecas
Los Mochis
Manzanillo
Matamoros
Mazatlan
Mexico City
Michoacan
Monterrey
Morelia
Naulcapan
Nueva Rosita
Nuevo Casas Grandes
Nuevo Laredo
Ojinaga
Pirules
Playa del Carmen
Puebla
Puerto Aventuras
Puerto Escondido
Puerto Vallarta
Queretaro
Reis
Riviera Maya.
Sahagun Hidalgo
Saltillo
Salto de Agua
San Blas
San Cristobal de Las Casas
San Jose del Cabo
San Juan del Ro
San Luis Potosi
San Miguel de Allende.
Santa Ana
Santiago de Quertaro
Sayulita
Taxco
Tijuana
Tlaxcala
Toluca
Torreon
Tulum
Tuxtepec
Uruapan
Valle de Guadalupe
Veracruz
Villahermosa
Viveros de la Loma
Xalapa
Yucatan
Zacapu
Zacatecas