Oh, one other thing...make her mix CD's (mp3 files) for the plane trip/successive trips. Up beat music works best.
And picture disks as well...slide shows of good times and/or a photo tour of the campus. People always asked what our campus looked like and it was hard to describe/get pictures.
Maybe a US map too, to illustrate where she's from. A sister sent me a Pennsylvania state flag, which was really nice
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I liked the idea of getting her the EuroRail pass! That's something a bunch of people could go in together on. It's much cheaper to get it in the states before going than to try to buy it there.
Along the same lines, look into the low-cost airline websites (EasyJet, RyanAir, umm...Jet2, Zoom, etc). Some of those have "gift card" options, which would allow your sister to purchase a flight to another country for a little side trip/backpack. The flights are dead cheap (i flew to italy and back for under $30 USD) and it was nice to have a break. A note of caution: it may be hard for her to get a flight unless she's living in a major city...check this first because some of these airliners fly out of some really random airports.