What's to see within a 2-hour drive of CDG Airport?

I stayed in Reims as an overnight stop on my way to Paris and really enjoyed it. Did not do a lot of 'tourist' stuff but the town was walkable and there was a Xmas market going on when I was there. There was a lovely park and we just did a simple dinner of our ever favorite fast food 'doner' . But it looked as though there were plenty of nicer restaurants to choose from.

Rouen, Amiens, Laon, Reims, Compiegne, Chantilly, all within a couple of hours' drive or less from CDG, all away from Paris.

For driving to Auvers-sur-Oise take the N104 just north of CDG direction west to Gercy-Pontoise till you see the exit to it, distance some 35km.
For Giverny you can drive further over the N104, becomes later N184 till it hits the A15 direction Rouen. Leaving the outskirts of Paris the A15 becomes the D14, keep following till exit Vernon near Magny-en-Vexin till you drive through Giverny. Total distance from CDG some 95 km, near Giverny make a little detour to Château de La Roche-Guyon.
Consider using a GPS and/or detailled map, especially for following other routes. Other places to visit north of Paris: Senlis, Pierrefonds, Beauvais.

With a car, this sounds like an opportunity to fit in the WWI battlefields of Verdun, which I have not yet made time for. Or by driving about 2.5 hours to the south, you can reach the construction site of Geudelon castle, being built today with medieval technology.

Although Guedelon is an unusual experience you'll never, ever find anywhere else, it will be over 2.5 hours if traffic is moving all the way, but even more if you hit a traffic jam. You have to go all the way around Paris on some congested roads. However, it could be an overnight trip, staying in Auxerre or Chablis, or even a weekend getaway during your month in Paris.

We stayed with relatives just outside Chantilly.. we borrowed their car and had a wonderful day visiting Chantilly and Pierrfonds.. Very easy drive about 45-60 minutes total each way on nice roads( we avoided highways) We didn't even have GPs.. we used dom de dom... "paper maps".. lol Worked fine!

Chantilly is one of my favorite places to visit ( been 4 times at least now).. its never a crowded nighmare like Versailles.. yes its smaller. but it has a WORLD class museum inside.. second best painting collection in france only second to Louvre.. and the gardens are a nice walk about.. be sure to visit the cafe in the gardens that specializes in selling strawberries and CHANTILLY cream dishes.. the strawberries will be the best you have ever had.. real sweet delights. and of course you are eating sweetned whipped cream in the place it was invented!

Peirrefonds is so picturesque .. like a fairy tale. .and once again.. while popular its still not mass mob scene .. Have lunch by the small lake for a truly beautiful view.. google for images. Pierrefonds was used for filming scenes from the series "Merlin" so you may recognize some parts.

There are many options to have a nice touristic "backroad" daytrip, I can suggest this one in the Vexin and Eure region north of Giverny, nice area with a pleasant vibe and lots of cute villages:

From the earlier mentioned D14 direction Magny-en-Vexin take exit 17 Dieppe, directly at the first roundabout take the parallel road to the D159 (vers D159) over 1km till you hit it and turn left to Guiry en Vexin and follow further to Arthies.
From there turn left following D983 direction Mantes-la-Jolie, go via D142, Villers en Arthies and D147 to Vétheuil. Take the scenic D913 along the right bank of the Seine to La Roche Guyon and go further to Giverny via D5.
From Giverny keep on the right (northern) bank of the Seine river to Vernon, follow D181 over a km for turning left to take the D313 to Les Andelys. Visit Château Gaillard there for a spectacular view of the Seine, in Andelys you can do a nice stroll along the bank.
From Les Andelys to lovely Lyons la Fôret (nice place for an early diner) via D1 , D2 and Écouis.
“Back home” via N31/E46, Beauvais and A16, total length of described itinerary to lyons la Fôret is some 85km.
If thinking this is too much, you can go back from Giverny via Ambleville to the D14 and if you like visit the Domaine de Villarceaux or follow the route along the tiny river Epte, total length about 60km.

Best is to use a well detailled map like Michelin’s Environs de Paris, scale 1/100.000 costs some €6 and practice a bit at home with Google maps and streetview.