Gate du Nord has lockers (you go through airport type security to get to them). GduN is where you will get the RER to CDG. Most museums open at around 9:30 am. On possibility for sun morning is the flea market at Clignancourt.
Yeah, did the Gare du Nord locker thing a couple years ago. 2.5 hour wait. Seriously. It was a Sunday, we arrive about noon. You have to wait in line until lockers have freed up, and when they let you in, you're in competition with others looking for someone leaving so you can snag it. It was quite bizarre.
I'm willing to believe that this was a particularly-busy Sunday (late July) and it's not normally so bad. But sure would be discouraging if it happened to you.
There is a
left luggage facility at CDG (operated by a private firm). I think that would be a much better choice (and keep you from having to haul the luggage on the RER as well...I think the moment you're "free" from your luggage, you'll feel much better).