M.R.
Some ideas (other than Theresa N's, which are good, too!):
Travel packs of moist wipes (especially if they are antibacterial)
Gift card/certificate for an online photo shop so she can have a travel journal/album made with digital pictures (my sister made one of her trip to Greece so she could put her notes in next to the pictures and it looked so nice!)
Extra memory card/s if she has a digital camera
Purse-size journal/notebook
International calling card/phone card
Barcelona is gorgeous so I'm sure she'll have fun and just appreciate that you even thought of her, no matter what you send! :)