I would not consider Macau a waste of time if you have never been there. Yes, there are newly developed areas that house casinos, restaurants, and shows. There's also a historic part that has ruins, churches, and temples. I think it's a really fun and diverse way to spend 2 nights. I think you would be short on time if you spend 2-3 nights elsewhere like Japan or China - which will cut into your Hong Kong time and not allow you to see much of the other country either.