I stumbled upon this useful post on Backpacker.com travel blog: What makes a good gift for a backpacker? Some would say cash works pretty well, but that's not very imaginative. One of those Amex gift cards would be pretty cool though--something to splurge with in a city somewhere along the way. Or a gift card to a travel gear store, virtual or real.
But since this is a travel gear blog, I linked to that post because they echo what I often emphasize here. The best gift, after a nice new backpack, would be something compact and super-useful. They go into Swiss Army knives, flashlights/torches, travel towels, and other good stuff. I remember when we were preparing for our first round-the-world trip and it was overwhelming to think about all the little things we needed to take care of before leaving. Having someone else take care of getting a few travel gadgets for us was really appreciated. Much better gifts than something we just had to put into storage!