When we went on our 9 month trip to Europe we got lots of stuff. Photos are awesome, we had them up in our room all the time. We also got a travel journal which I wrote in all the time and I have so much fun looking back over it. If she has a Pandora bracelet then perhaps you could get her the suitcase or plane charm for it?
I'd vote for either the St Christopher idea - either a pendant or a charm on a bracelet - or a nice leather bound journal maybe with her initials on it so she can write all about her trip.
Depending on what she likes, if you got her the bracelet then she could add charms to it from all the places she visits - probably not going to have a big budget as a 16/17 year old but if you got it in silver it might be workable for her. I have a friend who did the backpacking year in Europe thing after finishing Year 12 and the charm bracelet that resulted from it is lovely, as well as being a great reminder of all the places she's been.
A charm bracelet would be a great idea.
I like Sopdets idea of a little photo book, you can buy small ones that hold say 100 photo's, fill a few pages with family etc and she can add any others she wants PLUS pics from her trip.
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