Need road ideas for Paris to Amsterdam and back

If you really want »pleasant smaller towns« on the way from Paris to Amsterdam, I can recommend Mechelen (also known under its French pseudonym of Malines) and Dordrecht.
Historical Mechelen is strategically situated halfway between Brussel and Antwerpen, and has Gent and Brugge within a reasonable distance for a day trip.
Dordrecht is an underrated city, the oldest in Holland (in the sense of the western part of the Netherlands).
For staying in Haarlem I've seen positive reports on the Hotel Lion d'Or, at almost literally spitting distance from the railway station. There is a parking garage at two minutes' walk, but the hotel will know better than I.

We lived in Stuttgart for several months and the Mercedes museum is very interesting, with the oldest car in the World. Stuttgart is pretty much a day's drive from Paris. If you are interested, you may be able to go to the Daimler-Benz factory in Sindelfingen (sp?), south of Stuttgart.

If you are interested in WWI history, Verdun is on the way and worth a stop. Also, Strasbourg France is worth a full day. On the way back, you could stop at Trier, Germany and Luxembourg, en route to Belgium.

Like to add:

Just north of CDG Senlis (and nearby Château de Chantilly) is a nice historical town and even the castle is a 19th century rebuilt Pierrefonds is worth a visit to my opinion too. As is Arras with it's two large main squares more north along the A1 tolroad to Belgium.
Depending your interests Ieper / Ypres is the iconic Belgian symbol of WWI, completely rebuilt after the devastation but nowadays a lovely place to see.

In Germany the stretch along the river Rhine between Koblenz and Bingen.

Know that many historical places still have cobbled streets and Pierrefonds is a bit hilly, so can maybe an issue for you for walking around.

Arras is indeed an interesting small city, having seen much action in both world wars. Its architecture and cuisine are significantly different than southern France. The Vimy war memorial to the First World War is near Arras, celebrated by Canadians but architecturally impressive for anyone and with a revealing tour of the tunnels that were an important factor in that war. Moving north, Rotterdam is a major world harbour and a workshop for visionary architecture.
Stuttgart will take you away, not towards, Amsterdam, as you probably realize. There are plenty of places to visit as you drive from there north through western Germany. I like Aachen, once the seat of the Holy Roman Empire. But the trip north even goes through Luxembourg so there's lots to choose from.
By the way, if you are travelling trans-Atlantic, I hope you check out a multi-destination itinerary, flying into Paris and home from Amsterdam, saving time and maybe even money. However, a rental car that has to be returned to France will raise an expensive problem if you need to leave it in Amsterdam.

We definitely need to fly in and out of Paris since we're flying on buddy passes. That is the closest AA airport to get to Amsterdam. We could fly via London, but the flights to Amsterdam are expensive if we buy them at the last minute, which we'd have to do with buddy passes (which are no guarantee). The car rental we can get at CDG would be much cheaper than flying from London, and we've learned you can cancel car rentals at the last minute with Autoeurope if you book via phone. But most important, we love road trips and I find driving is the best way to get around due to my medical condition.

Pack a few Euro for the tolls - well, actually a few more than a few - and be sure that each driver has an up-to-date IDP, mandatory in France (unless you go to the hassle to get a certified translation of all drivers licenses), optional in Belgium and Netherlands, and if you drop into Luxembourg or Germany, optional there too. Don't exceed the speed limits - the Dutch invented (and bred, prolifically) the automatic speed camera, and the French have started scattering them about like salt and pepper. Be prepared for Belgian and Luxembourg drivers' habits. When you get to the Netherlands try the service area LaPlace cafeteria restaurants (they are neither cafeterias nor restaurants but I am at a loss how else to describe them to a North American audience). They are tasty..... and cheap.