A really good quality watch that she can wear for formal occasions. Jewellrey fashions change at that age, and she might outgrow them quickly unless you buy something timeless like pearls. But a good watch, that's for always. The watch I got for my 18th, is still the one I wear everyday because it was of such good quality. Simple, elegant, timeless.
At 18, you'd like to introduce them to good fashion sense, not tat.