France - Belgium Itinerary help

Find some way to fly on to BRU when you get to Paris. You are wasting a night, not to mention security line time.

Everyone here has different opinions of what are the best cities in that part of Belgium. Almost everyone fails to put Brussels at the top of their list. You can daytrip to Brussels from Antwerp (I have several times), so I would simply go to Antwerp and see the other cities from there. But I like staying in the same room for a while. It would be helpful to know the month of the year, and if you are student or adult budget level.

I went to Lille for the day from Antwerp, only to see the Palais de Beaux Arts (art museum.) It was three hours each way (local trains, not Thalys), so I don't recommend it. Lille's nickname is "Little Paris", but it is not the most important choice after the non-Paris cities you named already. Without knowing your interests, it is hard to urge that you sleep a night in Lille. There are so many SUBERB 1-2-2 1/2 hour daytrips from Paris that you can find a better way to use that night.

Edit: Please note that the OP wrote "I have to fly in and out of Paris". I always recommend open-jaw flights, but the OP made it clear that he cannot/will not do that. So I suggested a connecting flight. Even a two-segment flight on some national airline (which reduces the "penalty" for a failed connection, somewhere other than Paris, is even better. But the OP can't do that!

I have yet to visit Lille, but it is well recommended. It is a university town, with much history.

If you are interested in WW1 history, Ypres is a lovely town with so much history there.

A week is Paris is a good start. I myself could spend a month there every year.......

Have a great trip.

Dinant is worth a stop. I went when I saw the pictures.
Here is some info
https://en.wikipedia.org/wiki/Dinant

When are you going? There all kinds of great out door festivals from June to August in Europe.
You sight see in the day and party with the locals and tourists at night

I disagree with Tim about flying to Brussels on arrival in Paris. You cannot know in advance if your flight will land on time, how long it will take to go through passport control or how long a wait it will be for your luggage, if you're checking it. If you allow too much time before the flight, you'll have hours to kill in the airport when you could be sitting at a cafe looking at the Eiffel Tower or Notre Dame. If you don't allow enough time, you'll miss your flight to Brussels, forfeit your ticket and have to scramble to buy a (presumably) more expensive ticket on the next available flight.

After writing all that, I decided to see what the flight options are. The only non-stop flights between CDG and BRU are on Brussels Air and they only offer 2 flights a day, one at 7.30 am, the other at 8.45 am. So that's a non-starter.

5 days is not a lot in Paris. There are so many places to visit and things to do in the city itself, and half a dozen day trips I'd recommend. I haven't been to Lille so I can't compare. I've been to other French towns and cities, and haven't found one yet that I didn't like a lot. My only thought is that you are changing hotels a lot in the first week, so you may want to spend your second week in one hotel, no packing/unpacking, getting oriented to new surroundings . . .

If you haven't already booked your plane ticket fly open jaw -- into Brussels and out of Paris. It is smart to head straight for Belgium if you must do a round trip from Paris and end in Paris, but it would be easier and more cost effective to fly into Brussels and out of Paris. We have been flying open jaw for many years and it saves backtracking and is usually the average of the round trip costs into each city. On search engines it is 'multi city' not 'one way' when you plan. I would go to Belgium the day of arrival, booking the train in the afternoon to account for late planes if you are stuck with round trip tickets.

Are there still flights to Brussels from CDG? Air France puts its passengers on the train between the two. AF passengers have their own waiting room and luggage is checked through, just like for a flight. Anyway, it’s easy to take the train from CDG to Brussels. SNCF runs this route, not Thalys.

Janet nails the situation. Using a multi-destination search function can produce much more efficient itineraries than trying to piece together the various trip segments yourself.