Ok ok, let me told you something which guidebooks never mentioned!
I spent 1 year in the central region of Mexico, wanted to go Mexico City but was scared about the robbery/hijack. If you decide to go to Mexico City, be sure to go see the pyramid at Teotihuacan, it is worth the trouble.
Cancun and Acapulco are just like any other seaside resorts, not much different from Bali/Phuket, but be careful when someone approach you to buy those timeshare/resort holiday package. You will never need it. Just pay the day rate to stay in the resort.
One thing you never mention is how you intend to move around? I suggest you rent a car and drive yourself instead of relying on public transport. Public transport is cheap but no so reliable. Timetable is not updated and bus can breakdown easily. Rent a car from a reputable company like Hertz or Avis is better than a low cost local rental shop. Take note that Mexico is a big country, so between 2 cities people will fly instead of driving. Take for example, I drove from Guadalajara (central) to Puerte Vallarta (west coast), it looks near on the map but it took me about 8 hours to drive at speed of 140km.
For eating, you should note that Mexico food is unlike any other food in USA or elsewhere. Mexican food has a lot of meat and flour/pancake like. You may not like it if you are not adventureous.
Most Mexicans do not speak English, they speak Spanish which is a beautiful language. Probably about 1 in 2 men will carry a small gun either in the belt or inside their car. Dont mess with them or argue with them in small matters. And driving at night time is a bit dangerous, there were cases when someone stopped at red light junction, a robber just appeared and pointed the gun at the driver. So at night time, I would not stop my car, just drive slowly past the junction.
You can shop for many colourful handcraft in Mexico, they like to mix all sorts of colour, very attractive. Oh by the way, Mexican guys and ladies are very attractive, I always joked that every guy looks like Tom Cruise and every girl looks like Jennifer Lopez. They are not shy of being photographed so you could be lucky.
There are beggers in Mexico. I dont know your view of begging so to give or not to give is your choice, when you give, only give small coins, dont show up your whole wallet.
There is a well-known "culture" of Mexico which they called manana (pron. man-yan-la), literally it means "tomorrow". In real life it means never mind, dont bother, dont waste your time, etc. When Mexican says manana you can forget about it, they would not do it.
