“When I was in London bought a Sim card with 500 mb of data and 100 mins about $25 totally worth it for the ease of getting around and simplifying directions.” -S4H
“Sleeping bag liner. You never know how dirty some hostels/bungalos are. It’s also great if you’re in a super hot climate and just want a clean/thin sheet wrapped around you. Also makes a sleeping bag a few degrees warmer if you want to use it for what it’s designed for. Which I haven’t.” -Kalikoenig
“Microfibre Towel” -Many!
“A cotton scarf. So many uses. Turn it into a hobo bag, a towel, sun protection, dust protection, water filter, tie-down, eye-mask, bandage, arm sling, etc.” -ExpertVagabond
“Gaffer tape. Get the top quality stuff that is made for the entertainment and photography industries, not duck/duct tape. Why? The good stuff doesn’t leave any adhesive or residue behind. I’ve used it to hold a headphone plug in a loose jack in an airline seat, taped a book to a hotel vent to block noise, fixed a loose camera battery door, etc. Roll some around a couple of pens or pencils or other objects in your bag so that you have a meter or two. I roll full width around one pen and varying width narrow strips around another.” -tychosmoose
“Headlamp. Smaller than a flashlight, lights your path more fully, perfect for avoiding nasties on late night trips to the outhouse.” -nefariousmango
“I bought a Nexus 4 for $331 shipped to my house. Knowing that these devices were selling used for very close to MSRP, I figured it was a good investment. I used it oversees for one of my trips, came home, and sold it for $295. So technically this is $36 dollars. But this $36 investment allowed me to have (at the time) a cutting edge brand new unlocked, rooted smartphone. I tethered it at times to give my friend access to the net – which was nice. I think I went through $40 in data in Switzerland, $40 in data in Italy, and $20 in Dublin. After this though, I would highly recommend it. GPS walking directions? Yes. Google Translate? Yes. Pictures to help show people where we want to go (if our phone has no service or doesn’t work). Offline maps? Yes. Train timetables? Yes. Booking hotels/hostels while on the train? Yes” -factory81