You can make it more formal by saying "gomen-nasai" ごめんなさい or more friendly with "gomen-ne" ごめんね.
"Warui warui" 悪い悪い or "my bad" is also a very casual way to say sorry. "Sumimasen" すみません, which can be translated as "excuse me", also works as an apology depending on how it is used.
