A little outing to Paris

I’ve finally booked train tickets to go to Paris for a day next week. Can’t wait to finally show Magnus the Eiffel Tower. He’s been talking about it for so long. We’ve also booked tickets to see the dinosaur exhibition
It would be great if we also got the chance to have lunch at Café Coutume
The real reason for going to Paris is actually because Magnus needs a new passport, so we won’t have much time, but we’re going to make the most of it. Luckily my brother (who’s coming with us) is an expert on the Metro. 
I would love to go to Les Puces (flea market), but sadly it’s not open on the day we’re going. 
Have you seen the film Amélie? I love it! I can now watch in without sub-titles and I’m proud of it.  I don’t think I’ve heard French spoken at greater speed anywhere. We’ll definitely be visiting the café were it was filmed. Montmartre is one of my favourite places in Paris. It’s so full of life. 
Is there anywhere you would recommend? I would love to hear if there are places we HAVE TO visit, as this will be the last visit to Paris for a while… 
hx