The State of Mexico surrounds the federal district (Mexico City) on three sides. A large part of the Mexico City metropolitan area sprawls into the State of Mexico. Although small, the State of Mexico is a keystone of the central region of Mexico. It is bordered by several states. The State of Mexico, coupled with Mexico City, serves as the economic powerhouse of Mexico and is a huge population center.
Much of the state is at high elevation. For example, the capital city of Toluca sits at just short of 9000ft and a volcano nearby, the Nevado de Toluca, reaches heights of over 15,000ft and snow is common above 13,000ft. The altitude will be noticeable for most, causing drowsiness and possibly restless nights for a day or two. Aspirin taken 24 hours before arrival can help with the acclimatization, but always consult a doctor. Southwestern reaches of the state are lower in altitude and generally drier. Given the altitude in most parts of the state, prepare for temperatures reaching freezing at night during a large part of the year.
Spanish is the official language, but there are some native languages that can be heard here and there.
Nearly everything in the State of Mexico is connected to Toluca. The best way to access destinations is to use Toluca as a base. The best way to access Toluca is by bus or car. You can take a bus from Mexico City airport or from the bus terminal at Observatorio (the western terminus of the Mexico City metro). Your best option is to take a bus to Toluca and rent a car there, but if you must, you can drive to the western reaches of Mexico City and take the Mexico-Toluca tollroad (cuota). The highway between Mexico City and Toluca is one of the most modern in the country. Although modern, this highway can be quite hazardous as it climbs up to above 10,000ft. The weather is very unpredictable and often involves rain, dense fog, and hail. The often poor road conditions, coupled with fast-driving commuters causes frequent accidents, many times fatal. As long as you are aware and driving responsibly, you should be fine. Remember also that Toluca has a small international airport serviced by a few US carriers and Mexican carriers like Interjet, Aeromexico, and Volaris. Additionally, the State of Mexico can be accessed by highways coming out of the states of Queretaro, Michoacan, Guerrero, Morelos, Puebla, Tlaxcala, and Hidalgo.
Nevado de Toluca is an extinct volcano to the southwest of Toluca. You can drive to a parking lot at about 13,500ft that puts you about 1 km short of the caldera rim. Be sure that you are acclimated to the altitude before you attempt a climb. You should stay in Toluca for a few days where the altitude is just short of 9000ft.
Malinalco is a Pueblo Magico that is about an hour's drive south of Toluca. It sits in a green valley and the main attraction there is the ruins that sit on a mountain side that overlooks the town.
As a general rule, the closer you get to the border of Mexico City, the more vigilant you need to be. Some of the highest crime areas in the country are in greater Mexico City, which spreads into the State of Mexico. For a tourist, places to absolutely avoid would be Ciudad Nezahualcoyotl to the east of Mexico City (locals call it Neza, MiNeza, etc.), Ecatepec to the north, and Tlalnepantla to the north. These areas are notorious for gangs, kidnappings, and other violent crimes, as well as thefts.
As for the rest of the State of Mexico, a vast majority of the areas are very secure and calm. Toluca, the capital of the state requires normal precautions and awareness, as most cities do. Much of the State of Mexico is filled with traditional small towns where everyone knows each other and security isn't really an issue. Don't let the crimes of the cities deter you from visiting the wonderful places that this state has to offer.