yeah sundress is a great thing to get her. Just make sure you know her size and if you don't know for sure get something smaller than your first guess and get a gift receipt or something so she's not offended if you buy her something too big and so she can take it back if it doesn't fit. American eagle has really cute not to badly priced sundresses right now. Or you could always get her a giftcard.
actually I read sundresses are coming back in this SUMMER, I don't know if she's that kind of girl, but if she is she may know that they are coming back into style again for the summer. pick out one you think she might like, and or just bring her to one of her favorite clothing stores and let her pick it out herself, and if you're lucky she'll even model it for ya