Don't buy the BS that just because you have a kid doesn't mean you still can't be awesome in the gym. Don't think you have only have "three weeks left" too. I know you mean that jokingly, but still.

You might think you have an idea, but you don't at the same time on how drastically your life is about to change.....and all for the better, unless of course you are a complete D-Bag father who won't appreciate what is being given to him (not saying its you....but you KNOW the type).
So for your last three weeks? Stock up on sleep like someone said.

First two-three months can be VERY hard sleep wise. Sleep when they sleep during the day to help catch yourself up. Make sure you have a good camera and video recorder.

If you don't make use of it you will regret it.
And congrats on your soon to be living proof to the world you have had relations with a woman.
Seriously though. Cameras. Keep them charged and close by.