Unless you plan on taking your bag on long hikes, I recommend clamshell style backpacks. I have a 40L one that I have taken around the world and I tend to overpack. It is way easier to organize/pack. I also used a 55L amazon basic hiking bag and that worked perfectly fine. It can get a bit clunky and you might have a harder time sneaking it past discount airlines with no free carryon restrictions.
Things that have been incredibly useful to me
compressible bubble down jacket (like the patagonia micropuff but I got the Amazon basic one).
Convertible pants. You now can pack less pants/shorts.
Merino Wool sweater. Slightly pricey but great for layering. You'll be surprised how cold it can get at altitude.
Rain poncho/tarp. They are all over amazon and they fold down pretty small. You can use them in a ton of different situations and I have found them to be more useful than a raincoat (which is what I use to pack).
flashlight. Get those rechargeable ones they recommend on /r/flashlight.
a good battery pack for when you don't have easy access to electricity. It should be light/small enough you don't mind carrying it in your day back but large enough for at least 2 charges (I carry a 10kmah one).
if you are worried about weight/packing I'd advise investing in quick drying shirts/pants/clothing. You can easily hand wash your clothes and it will be dry in hours in most situations. 3-4 shirts, 3 pairs of pants, and extra underwear/socks should be about all you'll need.
carabiners and paracords are your best friend. They are incredibly versatile and useful. You can easily attach things to your bag or use the paracord as a clothe line.