I've backpacked around the world twice for a total of 12+ months, and while you may not feel that it's the most thoughtful, the BEST gift for is some money. When you travel like that you're really scraping by, often doing what you can to live on $10-20/day for everything, incl your hostel! Spending $20 on one thing (like a special piece of jewelry) is a REAL splurge.
I give money to my family who is traveling with a very heartfelt note to spend it on a very special experience/item that they may not usually treat themselves to. Journals/backpacks/travel things etc are nice, but she probably has everything she needs. We received a beautiful Natl Geog. travel journal, but that thing weighed about 2 lbs so we left it at home. I have a pile of travel gifts people gave us that never made it out of our house. You have to be really picky when you're backpacking!
Go the money route, but add a very special note about how she should spend it, I love all the "expensive" (i.e. $10-20) jewelry pieces I've bought on my travels. :)