Unless you can think of something in particular that she'd really like, always go for jewellery! Most people adore getting jewellery as a present, I know I do! Probably best to go for something silver or gold rather than costume jewellery - I got a lot of costume jewellery for my 18th which I loved, but I think silver or gold is more universally accepted as an 18th present. It doesn't need to be from an expensive jewellers', it just needs to look pretty and classy. Go for a nice necklace - matching earrings and maybe a bracelet are also always good. Or you could get her a charm bracelet and buy a couple of charms that are particularly significant - then every birthday or Christmas you can buy her another charm to add to the bracelet, making it a very personal and cute present!