About 30 miles outside the centre of Mexico City lies the ancient city of Teotihuacán, which was first established in about 100 AD. The origins of the city and the people who founded it (the name Teotihuacán, meaning 'birthplace of the gods', was given to it many centuries later by the Aztecs) remain mysterious. However, the site — famous for its two large pyramids, the Piramide del Sol and the Piramide de la Luna (Pyramid of the Sun and Pyramid of the Moon) — is one of the most popular attractions in the Mexico City area.
You could take an organised tour to Teotihuacán, but it's much cheaper and, I think, better to make your own way there, either by car or from one of the frequent buses from the Terminal Central del Norte. My return ticket cost me 88 pesos (under £4) and the journey took just under an hour. NB: you can buy an open return ticket if you don't know which bus you will take on the way back. I arrived at the site just before 9 am, which meant it was less crowded and less hot. I also opted to go on a Monday — partly because most of the museums and attractions in Mexico City are closed on Mondays, but partly because I had read that Teotihuacán would be quieter on Mondays.
The first thing to note is that Teotihuacán is really big. There aren't many signs or maps on display either (presumably to encourage you to hire an official guide), but my guidebook had a useful map of the site, which helped me to orientate myself. I arrived at gate one, which takes you in through the south of the site, nearest the Ciudadela — a large, square space with low walls, not dissimilar to Great Court in Trinity College, Cambridge. It's then about a 15-minute walk to get to the Piramide del Sol, which, at over 70 metres tall, is the world's third tallest pyramid. You can climb all the way to the top and it's a very steep climb, with only partial guide ropes, so do wear trainers or other suitable shoes.
Once you reach the top, you realise the climb was worth it. The views across the whole of Teotihuacán, the Piramide de la Luna and of the surrounding valley are stunning, and you can't help but marvel at the achievement and, in my case, the geometry. And yes, there was plenty of time for some leaps!
After scrambling back down to the bottom, I walked along Calzada de los Muertos ('avenue of the dead' — the Aztecs thought the smaller pyramids that lined the path were tombs) to the Piramide de la Luna and, you guessed it, climbed up that one too. Luna is slightly smaller than Sol, but the former sits on higher ground so there isn't too much difference. Based on the position of the sun, my photos came out a lot better on Luna too. If you're interested in the design and construction of this and other ancient Mexican sites, this blog post, which popped up somewhere in my RSS feed this week, has a lot more information.
Afterwards, I visited the mural museum, and then the site's historical museum — the former had little information, but the latter was quite interesting. There is also a small botanic garden and a picnic site, where I ate my packed lunch. By the time I left, around 12.30, the place was starting to get busier but I was still amazed by the fact that I could have one of the mini-pyramids to myself to sit and reflect in peace.
My guidebook said to go back to gate one to catch the bus home, but the bus ticket said gate two, so I went there instead (a slightly circuitous route, but pleasantly quiet). There is no sign for the bus stop but if you go through the gate and cross the road, the bus will be along soon (as you face the site, the bus comes from the left). The entrance fee to Teotihuacán is 64 pesos (just £2.70), and I spent just over three hours there. If you're in a big group or walk more slowly than me, it could easily take a bit longer to visit all of the site's sights. If you do have a spare morning while you're in Mexico City, though, you should definitely visit Teotihuacán. It's well worth the effort of getting out of town.