I'm going to do my manager's job when they're away
Why are you thinking about doing your manager's job?
I think about doing it a lot, but I'm afraid of the consequences.
It's okay to help your boss out from time to time if they need it, but make sure you don't get taken advantage of. You should also be sure that you boss wants you to do their job.
I'm going to make a complaint.
It's right of you to make a complaint about your boss if they are expecting you to take on their responsibilities without fair compensation. Doing this will ensure that your boss does what is expected of them.