I intend to return to Guatemala probably in 2016 (I'm booked this year), and when I do it, it's my intention to take a 50mm and a 90mm with any of my meterless Leicas. The last time I was there was in 2010, and I had a huge Nikon D700 with me. I didn't stand out that much in Antigua (everybody carries cameras there, it's the tourist ID), but elsewhere... In any event, in Xela, a small, unobtrusive camera should do the trick.
Now, since you have traveled all over the place (if memory doesn't fail me, you went to Perú recently, and then you had been to Ireland and other places in Southeast Asia), you'll understand my logic here... To me, Europe is a wide-angle place: narrow streets, atmosphere all over you, and then a 28mm or a 35mm lens are perfect. Spanish America is for narrower views, which means the use of 50s and 70s and even longer lenses needed to shoot portraits or isolate details in houses, streets and markets. With my digital Nikon I had an AF-S 24-120mm zoom. I recall using the longer-ish end of the zoom a lot more than the short one... but then, that may be just my style. Every time I've been to Europe I found that my needs are met by my short lenses, and even though I've traveled with my Summicron 90mm, I only have pulled it out of the bag for... what? Maybe a dozen shots...
If wondering around with the M9 makes you paranoid, leave it behind. I think I'd take it, simply because it'd be an excellent travel camera. Now, if you also have the X100, bag it as an alternate body.
Café Antigua is now a coffee type, and it's fairly available in Guatemala. If you're adventurous when it comes to eating, try tamales and chuchitos; they're both variations of corn dumplings (the former is large, the latter small) with meat of some type (often pork) and a sauce. The best beer is called "Gallo" (literally "rooster"), and, in Antigua, an excellent restaurant is "La posada de don Rodrigo" (it's been in business since the early 80s); just beware in this place: they don't take credit cards. Otherwise, they're widely accepted, although I would never use them for anything below the equivalent of $30.00 (there are currency conversion hidden charges for every transaction). Oh, and you can bargain in markets, when it comes to purchasing handcrafts. The best strategy is to offer half of what the seller asks, and then continue until you make it to an agreement. If you simply don't want to buy anything, ignore the offers you hear (they can be quite persistent).
Take care and enjoy! You'll be there in March, which can be very warm if you stay in the capital city, but fairly pleasant in Xela. BTW, how are you going to make it there from the airport?
