Speaking as someone who's sold both boys and girls, ranging from late teens to people in their mid twenties, on Sentai and Kamen Rider before, the best advice I can say is tailor your suggestion to the person you're trying to find a series for. Who they are, not their gender.
That said, I'm gonna say Magiranger and Shinkenger are pretty good suggestions overall. You'd have to compare them to your...um....target's....interests to be sure they have the maximum chance at success, though. Overall they're good picks, though. Not too heavy on story, but it's still there and in sufficient force to keep you interested.
Goseiger...I suppose I am slightly biased, although I'll agree that from a cosmetic standpoint someone who's never seen any other Sentai won't care too much, but it may not be good if the girl is older and/or requires more plot in her things that which she views. (yes, the horrible syntax was on purpose. I'm weird like that) The fact that they never have a concrete villain team, and some of the lack of answers and explanation early on may throw her off.
I will, however, suggest Boukenger if it's an older girl. I suppose Eiji and Akashi are reasonably handsome and all, but the feel and theme of the series is just very broad and I feel like it may appeal the most to anyone, male or female. It's not just boys that enjoy a good adventure.