I also use a tablet/portable Bluetooth keyboard solution. The form factor/weight of my old Nexus 7 works much better for me than an iPad Air for international travel. In a pinch I can draft on my iPhone.
I use Dropbox for my ongoing book project. Some parts of that project require a laptop, but I work on smaller bits while traveling. For those, the Nexus works just fine. I am careful to download the in-progress drafts to my tablet before I go (and test that I can access it offline.) I have DocsToGo for word processing, though it's clunkier than I like.
More often, I use travel as a break from my manuscript, and I focus on new work. I carry a small Moleskine wherever I go for quick notes. I use my writing time (or train time) to journal heavily, and I often do this in Evernote, because I'm just laying down material and not editing. It's easy to cut and paste from Evernote when I return home, and then clean up the draft. (Another option is to email these drafts to myself.). I get several essays from each European trip, so the time spent taking good notes or journaling in detail really pays off.
Just started re-reading Eric Maisel's book "A Writer's Paris." The book inspired me on a 2006 tour in Paris, especially the advice to approach travel as "more artist than tourist." In 2014, my teen son and I spent an "artist's morning" at Place des Vosges, a la Maisel. A wonderful experience. The book helped me transform how I travel as a writer.