What to pack!?

[neuroanatomist]

An ultrawide like the Canon 10-22mm plus a 50mm prime (short tele on APS-C) does leave you without a 'normal' lens - maybe that's ok, but I'd be tempted to bring the 18-55mm. As long as you have ready access to your friend's 100-400, leaving the 70-200/4 behind makes sense. But you'll want a telephoto lens along, IMO.

People associate wide angle lenses with landscapes, but many times it's very effective to be able to isolate elements of a landscape. Also, if you've never used an ultrawide lens, they take thought and planning to make the most of them. Just shooting a wide shot to capture the majesty of a scenic vista almost always produces disappointing results that don't measure up to your memory. Instead, have a very close subject in the frame to draw the eye into the image.

I know you asked about lenses, but also be sure you have a decent tripod!

[Tounis]

People associate wide angle lenses with landscapes, but many times it's very effective to be able to isolate elements of a landscape. Also, if you've never used an ultrawide lens, they take thought and planning to make the most of them. Just shooting a wide shot to capture the majesty of a scenic vista almost always produces disappointing results that don't measure up to your memory. Instead, have a very close subject in the frame to draw the eye into the image.

I think what neuro said is especially true in the mountains. You'll probably be impressed by the wonderful mountains in Peru, but if you only take pictures at 10-15mm, they'll end up looking very small and unimpressive on your shots, especially those in the distance. Your pictures will not represent your memories at all.

I live in a quite mountainous region, and when I had my 450D, the 15-85mm was great for this kind of photography. It's wide enough on the wide end to allow you to take nice landscapes if you get an opportunity, and the 85mm on the tele end gives a lot of versatility to play with perspective. The only downside is that this lens isn't fast at all, but the 50 1.8 could be there if you need a fast lens.

I hope you'll have a great trip in this wonderful country.