Plane, of course.
Airplane or first-class bus line should be the preferred way of travel, at least for tourists visiting Mexico.
A great website that tells you the best way and time to travel to Baja, Mexico on a budget would be at gomexico.about.com/od/.../f/when_to_go_mexico.htm. This website gives you great detail on which way and the best time to travel to Baja, Mexico with a dog on a budget.
What is the best way to travel from Innsbruck to Salva
The Best Way to Travel was created in 1968-07.
Mexico City is IN Mexico; it is the capital of such country.
Your passport is the best one. On your way back however, you may also use a birth certificate or ID. The problem is not entering Mexico; it is going back into the United States.
Bus lines are pretty accessible to local and foreign tourists alike. If you prefer to pay a higher price for a shorter travel time, there are some charter flights from Cancun to Merida.
As long as you have an American passport, you can enter Mexico. Problem is on your way back to the US. Without a valid proof of citizenship, you will not be abe to return to US soil.
Their are many travel websites that can offer great deals for trips to Mexico such as Travelocity.com and Expedia.com. They may even have last minute promotions to Mexico.
December 1 trough May 30th - out of the hurricane season.
boats
To fly into Mexico you HAVE to have a passport. As of right now the only way you can get into Mexico without a passport is by land travel, but that will change as of Summer 2009. As of Summer 2009 you will need a passport for any land travel into Mexico and Canada.