I think that's the problem with 'once in a lifetime' trips. Trying to do too much in too little time. I'm also not good at it, but I am trying to learn to be satisfied with doing two or three things indepth and fully enjoy it, instead of doing eight or ten things superficially.
Like with the Louvre, it really depends on what kind of person you are. You have the tourists who go in, snap a selfie with the Mona Lisa and then leave again, just to be able to say they saw it. The tourists who love art and want to see every artwork in a relaxed pace. And everything in between those two. In retrospect, on my first trip to Paris, I should have gone to Musee D'orsay, as the impressionists interest me more.
Same with the Notre Dame, I am not a religious person, for me the building does not have the significance as it has for some people. I did enjoy the architecture and of course standing on the square in front of the Notre Dame while listening to the opening number. For the Notre Dame I do not need more than an hour or so. I am not a fan of viewing points, I don't really care for it, unless it is my own city and I recognize things myself. When I stand there and see on the sign on the viewing deck that the tall building on my left is the 'such and such building', then all I think: 'ok, next'.
So I didn't go up the towers of Notre Dame, and with the Eiffel Tower, I didn't need to go up this tower either. For me taking a picture while walking by it was enough.
What I am trying to say is pick things you love and that interest you, don't see things in a city because they are on the highlight list. You can go off the beaten path on your first trip.
Back to the question what it's all about, in twenty years from now, would you kick yourself for not going to Disneyland Paris? Is there a chance you will say 'I should have done that?' If the answer is yes, then go to DLP