A few years ago I received a Midori Travelers journal, which I LOVED because it was refillable. It's a beautiful leather flap which you can customize and they sell high quality paper inserts, including watercolor tear out pages. I wish I had this when I was young and backpacking around Europe and Asia, because I could have kept using the same notebook but with a new insert for every country or continent. So fun!
I did discover one slight flaw though, and that is the layout. It's long and narrow, which makes for some pretty skinny paintings. It's alright for landscapes though, which is probably what I paint most while traveling anyways.
Here are a few of the paintings that I did when I was first getting into watercolor and en plein air: