I think I would say something like : 你今天特别漂亮啊！

i think it depends on how handsome you look. if you are pretty, no matter what you say, girls will like it. however, if you are not, just be noticed that Asian girls don't like being praised directly, although we may be flattered on surface, but we just think that this is too abrupt, and this guy have some underneath purpose.

你今天真美

你今天看起来气色不错。（You look fine today.) 你今天穿得衣服（clouthes）真好看。

I think it is up to what actually do you want to say. If you want to say her appearance is good today,you can say"你今天很漂亮" If you want to say her status seems well today,"你今天的状态很好"is ok