(1) For int'l travel I always print out itinerary / confirmation emails and save paper boarding passes. Many times during exit immigration this has been helpful, esp when there is no entrance stamp in my passport or it is illegible. And can be useful during entrance immigration if they want proof of onward travel.
For domestic travel I never print itineraries and use either phone or paper BP depending on which is more convenient.
(2) I always bring small pieces of paper (index cards, or a small notebook with removable pages). Great for writing addresses to hand to taxi drivers when there's a language barrier, leaving notes at hotel desks for family members, jotting down train times, etc., etc.
(3) I always bring a portable battery pack charger for my phone (duh...never though to do it until I got one and now can't leave home without it). Usually don't have to use it but always helpful during those long delays or when the charger falls out of your phone and you don't notice till you wake up and your phone is at 10% battery and you have to rush off to the airport. Mine doubles as a flashlight too which I've used many times to find things in my bag, look under the bed, etc. I guess most road warriors carry one at this point but I find many of my friends don't.
(4) Along those lines, I often carry a three-to-one power outlet splitter like this:
Costs $6. When travelling int'l I only have to carry one power adapter and can plug in 3 devices. When travelling anywhere with US-compatible plugs it solves the problem of when you are at the gate and there's only one outlet and it's already been claimed -- no one minds unplugging their laptop for 2s to let you plug this in and then more people can share.
(5) I guess I'm obsessed with power...but a long time ago I bought a phone charger and laptop charger with Euro plugs and I take them when I go (and leave behind my US ones). Way easier just to plug/unplug native chargers than carry flaky power adapters everywhere.
(6) I always always always carry a Clif Bar or some other kind of granola bar. Never know when your plane will sit on the tarmac for hours, or you'll arrive in a new city late and everything is closed. Travel delays are bad enough but if you are hangry they are worse. Even just knowing I
could eat if I needed to makes things easier. If it's someplace with a warm climate or in summer I always carry 500ml of water too.
(7) I carry a small ziplock of tea bags of my preferred brand. Small, light, and cheap, and means I can have my morning beverage wherever I am. Hot water is widely available but good tea often is not, and I don't drink coffee. (Totally happy to have the local tea if it's good though!)
(8) Ok now a weird thing...when packing I usually just take shirts, pants, etc. on their hangers and fold them loosely into my suitcase in one big bundle. I don't take them off the hangers or fold them individually. When I arrive on the other end I just open my suitcase and everything is ready to hang up super easily. I find things don't get any more wrinkled than they do when folding individually, and I'm more likely to hang things up quickly upon my arrival (and I don't have to go hunting for hangers, etc.). Things too dirty to be worn again go in the suitcase off-hanger (but I keep the hanger around), so every place I stop I just re-create a mini version of my closet at home basically.
I even do this while (non-backpacking) camping, I just hang up my clean clothes on a tree or on my hammock rope.