She's 19 and has a Mac - that doesn't mean she wants computer gifts unless she's pretty geeky.
How about the body lotion for whichever scent she usually wears? The stores are full of coffrets at the moment where you get freebie extras too if your budget can stretch to that. Women occasionally treat themselves to perfume - they rarely spring for the extras so it's always nice to get them. If you've got a department store selling Origins, go look for their limited edition Ginger Glimmer body lotion - it's amazing stuff.
Or fashion jewellery (Murano glass hearts on thongs etc) perhaps.
If you get really stuck, buy her a gift card for a local department store so she can put it towards something she wants rather than having to say thanks for something she doesn't really want or need.