Well you be paying hundreds of Euro in a drop charge in a different country? The car has to be returned to France before it can be rented out again, so you will pay a high fee to pay for that.
Or are you planning on returning to Paris?
Do you know about the tolls?
You can go by Thalys train all the way to Amsterdam for as low a €66 combined for the two of you.
To break it into a combination of journeys will take me a little longer.
Are you under 26 years old?
Most hotels in Gent charge from €15 to €25 a night. What level of hotel are you looking at?
The problems with driving from Gent to Amsterdam (I have to do sometimes so I have experience) are the heavy traffic on the approach to Antwerpen, and then your decision to go clockwise which means expensive tunnels, somewhat complicated route finding, and then heavy traffic all the way through the Randstad; or anti-clockwise which means all the extremely slow traffic around Berchem and difficult highways through the centre of Antwerpen, and then the heavy traffic all the way through the Randstad.
Parking in Amsterdam is difficult, rare, and expensive, and then there are all those bicycles which have the right of way. Can I assume you will return the car, maybe at the airport, before going into town?
The Dutch invented the Gatso - the speed camera, and derivations of it, and they are absolutely everywhere in the Netherlands. A lot of the highways now have variable speed limit gantries - they reduce the speed and have cameras on the back side of the gantry - slow down before the gantry, don't take the new speed limit as the place you take your foot off the gas.
Driving in Gent is interesting because of all the ongoing road works on both the highways and in town. The hotel ring often has diversions (sometimes well marked, sometimes not) because of all the road and building works. It is best unless your hotel provides parking to park in one of the major car parks on the Hotel Ring. Rapp, or Vrijdagmarkt are often better than others.
The drive from Lille is fairly straightforward once you have left Lille. And before you get to Gent. Straight flat fairly uninteresting motorway.
Driving in Lille is like driving in Belgium. You need to expect no signals on cars before changing lanes, cutting you off and driving in your back seat. The speeds on the roads around Lille are restricted and variable so you often have enough time to see where you need to go. I stay in a roadside chain hotel in Englos, a suburb, but there is no bus or tram or train within walking distance. Parking is free, but to get into the centre you need to drive to a station at the end of Métro (free parking if you can find it) and then ride that into the centre.
I don't know much about Montreiul-sur-Mer - nearby yes, but not the town. The official toll website shows a cost of €36.99 from the CDG airport to there. That's a toll of €16.40 and probable fuel use of €20.59.
You should know that many of the Gilet Jaune protests started as protests against fuel cost increases.
The train, with this much notice, can be quite reasonably priced.