Give a gift certificate is always a little easier: you do not need to rack their brains to find a personal gift. On the other hand, a gift certificate allows the recipient to crack the few things she would probably not normally purchased. And she's sure to have fun without the risk of ending up with a book or CD that will unnecessarily cluttering the shelves. She will not resell it for half its value on ebay!
The gift certificate is Amazon provides a nice pouch with space for a dedication. It can be sent directly to the person or at home if you want to put under the tree. It can also be emailed or printed at home for those who would take it to the last seconds.
Top Amazon would pair this check with a charity (eg. 50 on the gift certificate, Amazon would repay a few euros to an association).