What should I know before travelling to Mexico?
Basically all that Elena said is true.
Bring very light clothes. Although with the humidity it still is hell. Bring mosquito repellent, a hat and sunblock. Very important. Also, people here tend to kinda dress up a bit for social functions (even if it's just a meeting at a friends house), have a couple of nice clothes along for that use. We always make fun of americans in shorts and sleeveless shirts going to bars (we do dress up). For going out at night you need something nicer.
Something that it's kind of universal in Mexico is the need to have cash. A lot of business do not accept cards. Also, tips and street food and other stuff need cash.
Have a local cellphone, try to locate much free wifi hotspots as possible (there are quite a few in the city parks). Data usage in prepaid phones is expensive, and for the time you are here it's not worth it to get a plan
For Elena's point No. 1, the place that guides will try to take you is called 'La Chaya Maya'. I recommend you try to go to 'Los Almendros', it's a better place with better cuisine and it's not jampacked with tourists. But seeing that you have a lot of time, you can get to know a lot of food.
Food at mercados (the market places), tacos, there is all sorts of cuisine.
If you have a very restrictive diet like vegan (vegetarians can manage) or kosher or halal (most people have no idea this exists), most restaurants do not have options for you, and you would have to figure out the logistics of eating at your place. There are like 1 or 2 places for you at the most.
For point No. 2, If you go to Chichen Itzá (it's actually closer to Can Cun than Mérida), it's pretty much a day trip. Do go to a cenote to take a bath. X'keken is close by, at is very famous.
- Food. Merida has one of the most extensive, regional cuisines of Mexico. It´s famous all on its own. If you are interested in Mexican food (the real one, not the American variety) you´re in for a treat. Be careful with the habaneros.
- If you have time, go visit Sotuta de Peón and Chichen Itzá.
- Go watch a regional show, where they have Bombas.
- Try the limeade.
- Have an ice cream on Paseo de Montejo. If you ride a carriage, know that the horses are well cared for and work only for a limited time.
- It will be very hot. Very humid. Because of that, people in Merida don´t function by normal hours. They rise very early, hide in their homes for the worst of the heat, and come out again in the evenings until very late. You might find stuff closed at midday.
- Merida is one of, if not the most, safe, relaxed place in Mexico.
There are several different aspects to consider before you go, from travel documents to health and safety concerns, and of course which destination to choose and what activities to pursue during your stay. Here are some resources to help you get your trip planned and figure out what you need, where you should go and what you should do, to help make your Mexico vacation a success.
I recommend you ask your cell phone carrier for charges during your stay in Mexico. If the fees are too high, I recommend you buy a SIM card with Telcel as soon as you arrive, and get a prepaid plan with data and voice calls included.
You will need this because I assure you will make a ton of friends.
Have a lot of fun. You will never forget this experience.