「今日はきれいですね。」 if she is a girl you just met. (Polite form) 「今日はきれいだね。」 if she is a very close friend. (Casual form)

今日可愛いね(・∀・)!! 今日かわいいじゃん(kanto dialect) 今日かわええやん(kinki dialect) Japanese girls like to be paid attention, but Jpnese guys usually don't say things like that. Maybe only womanizers do? haha Good luck to you