I really love what they did with the interplay between Aqua's femininity and her strength because they went a lot farther than simply giving her girly traits to keep her from seeming overly masculine.
Her two biggest feminine traits, I'd say, are gracefulness and creativity... which she embraces wholeheartedly, then channels to make herself a more effective fighter. They're things she uses as a source of her strength, rather than flaws that weaken her.
Then, she's got another set of traits that could be seen as feminine, but have alternate connotations that are much more pertinent to Aqua in particular. She's bound by her duties and her Keyblade summoning involves cherry blossoms, but those traits evoke the image of a samurai as much as, if not more than, the image of a woman. And, likewise, she's empathetic, sentimental, and shows great concern for her friends, but in the context of Kingdom Hearts those traits are preferable for everyone regardless of gender.
And, to complicate things further, she's got a third set of traits that are either not feminine, or just show a lack of adherence to typical gender roles. She's chivalrous, practical, has a great deal of control over her emotions, and appears to be entirely unconcerned about maintaining a perfect appearance. She may be attractive, but she's much more likely to project an image of ferocity or inner strength than of beauty, and she's not afraid to move in ways that seem a bit masculine if it makes more sense (for example, the way she carries Kairi under one arm to run).
She's one of the few characters I've seen who manages to completely avoid the double-bind of either being too masculine or too stereotypical, and for that, she's awesome. ^_^