I have two daughters of a similar age - both students, I always buy them their favourite Mascara, Micellar water, some kind of body spray - this year they both have Sabrina Carpenter, and a Sundae shower foam - a little expensive but a nice treat. All of these will be used and save them money in the long run as they won't need to buy them. Last year I also put an Elnett hairspray in too.
I also put in a Chocolate Orange, choc coins and something fun, so this year I've got them both a MiniVerse lifestyle ball thing, last year was different UNO card games, a little childish but something to do on Christmas Day, I've also splashed out and got them each a JellyCat tree decoration to put in the top.
TKMAXX is a good place to look, they have lots of unusual/quirky things that are generally good value.
Silk/satin hair bands, nice face wash and cashmere blend socks (you can pick these up in primark for a fiver) also go down well. Also Tweezerman tweezers - to stop them stealing mine!
I find that you can end up spending an awful lot of money on stockings, so I try to make sure that the majority of the things I put in are consumables that will definitely be used, with a few funny little bits that the girls won't be expecting.