Teotihuacan is a definite must-see close to Mexico City! It consists of an extremely large (but characterless) 'city' area punctuated by two large pyramids, which are really the main attraction. The larger pyramid is climbable all the way to the top, for people of all ages (toddlers to senior citizens). The climb is fairly (but not extremely) strenuous, and stairs can be intimidatingly steep in parts, but the view is worth it. [And ACTUALLY, the smaller pyramid has the better view (even from part-way up), so don't skip it!] The larger one's climb takes about 10 minutes if there is no crowd. If there IS a crowd, expect to get frustrated ascending and descending in a line, and it may take 20 minutes. I was there on a non-holiday Sunday (Sundays are free admission, by the way, although usually less than $5USD anyway!); we physically got to the ruins around 12 noon, and there was a 'tolerable' amount of people there. The line was moderate. It seemed to get more crowded after we left.

As with the rest of latinamerican tourist spots, expect MANY vendors within the ancient city's walls. In addition to those who are allowed to follow you around the ruins, there is a market area and bathrooms at the bottom of the large pyramid -- silver, obsidian, and general handicrafts are sold.

RECOMMENDATIONS:
-go on a WEEKDAY, or IN THE MORNING. (Ruins are always more majestic when you can see them at your own pace, and catch a photo without others bustling through.)
-SUNSCREEN is a must -- you will get horrible tanlines and/or sunburns, even after an hour. :)
-I would NOT recommend spending money on a guided tour. Little is known about these ruins, because they had been built even before the Aztecs happened upon them. You will only hear a lot of "this MIGHT have been for this," and like I said, there isn't really much to tour anyway, besides climbing the outside of the pyramids.
-Allow two hours to meander around and climb both pyramids -- a little more if you want to browse shops or walk down the less-interesting paths.

Bottom line: definitely not as impressive as Peru's hidden Machu Picchu, but still breathtaking in its own rite, despite nearby modern civilization that is visible from the temples' tops. Plus, it's only 35 - 45 minutes from downtown Mexico city. You could set up a tour through your hotel, but I would go own your own accord if you are more comfortable doing your own thing.

Awe-inspiring is the only way to describe Teotihuacan. My family and I visited the place recently and it truly makes you think about the type of civilization that produced such an enormous and majestic place. What makes it even more fun for the traveler is that because nothing definitive has yet been established about the civilization that built this area nor their reasons for abandoning the place, you can let your imagination take over as you walk around the area. If you go, remember the following: do not go on your first or second day in Mexico City especially if you are not in top physical condition or if you are not used to being at high altitudes. It takes time to adjust to the altitude of Mexico City. To climb the pyramids will require stamina but once you get to the top - it is worth it. Go as early as possible to avoid the crowds and the lines to climb the pyramids. If you can afford it, pay for a good tour guide to take you there. Contact me off-line if you want a recommendation. Bring sunscreen and water. Use comfortable walking shoes. I could not understand why some people would want to climb the pyramids wearing high-heeled shoes.

I made the horrible decision to drive in Mexico City. I have driven all over the world, and NEVER have been so frustrated with ANY city. Focusing on Teotihuacan; When driving North from Mexico City stay on what may appear a main road, I know it'll be hard, but don't be fooled by the splits on the road. Follow signs that say "pyramidas" or look for the little picture of a pyramid. The road dead ends into the pyramids, so don't get spooked that you missed a turn. At the toll booth, if you stay to the extreme left, you miss paying the toll, go figure, the second toll booth was unmanned, so just drive right on thru.
My second suggestion for the actual ruins, is to drive around back. When you enter thru the main entrance, there is a big lot. This lot is at the far end of the ruins, and there is much climbing over large barriers. You will notice a road that circles the entire area. Take it. Park by the pyramid of the sun, then drive back to the main entrance, and see the temple of Quetzalcaotl. This is by far the most fascinating part. They are restoring this part, and it is the only place to see sculptures not in the museo.Placed right where they used to be. It gives you the idea of what the place looked like back then. A MUST SEE.
The vendors are let in to the ruins to ruin your visit. our best bet if we didn't want anything, was to just shake our head no, and not acknowldge their existance. If you say something, they will follow you around. This is the only reason I didn't give this place an excellent rating. These people are EVERYWHERE inside this place, not outside the gates where they should be.
The tour busses arive late morning and early afternoon gets crowded. So go early, they open at 7 AM. (close at 6PM)Admission is free on Sundays, but that would be a foolish day to go, as everyone in Mexico City enjoys saving money.

I've been to Teotihuacán 3 times now. In my opinion, it is one of the wonders of the world...once the largest metropolis of ancient America. One of the things that I absolutely love about Mexico, is that you are allowed to get up close to historical treasures like this; you can climb both pyramids and explore most of the ruins. Here in the U.S., government regulations or the threat of lawsuits would no doubt prohibit us from such activity.

There is a decent little museum on the site (el museo del sitio), which is often missed by visitors, as it is off to the back of the Pyramid of the Sun. Between the Temple of Quetzalcoatl & Pyramid of the Sun, we ran into a staff member, who showed us the mica depository...very interesting.

One of my favorite things about Teotihuacán is the vendors. There are a multitude of them all over the site. My advice: do NOT buy from the first vendor, even if you really love the item. You'll see the same item again somewhere on site. Do take notes of the prices they are asking. My strategy is to look at things from 3 or 4 vendors, note the prices, and let them know that the last guy wanted x pesos for the same thing, and ask what his best price is. They almost always ask a really high price. Even i got taken on my third visit! My friend bought what looked like a little harpsichord string instrument thing for 250 pesos. I wanted one for my classroom, so I asked the next vendor how much; he started at 350, but I got him down to 200 and bought it. Wouldn't you know it, on the way to the museum, another vendor holding one shouted out "150 pesos" as we walked by. Go figure. I probably could have bought one for 100 pesos.

If you see something you want, check the shops at the entrances, and see what they charge. The vendors will often give you a better price. Some of the vendors are very persistent, but that's how they make a living. Be polite and enjoy the game!