one day/night near CDG- suggestions for place/activity?

I'd probably spend the day in Paris, even though you've already spent 5 days. There's a million things to do there and even if you just stroll around it'd be lovely. But I can never get enough of Paris!

I believe there is bus transportation to Senlis (reportedly very picturesque) from CDG.

Where are you staying the night before? Frankly, I’d walk them to security, wave goodbye, let my goodbye-pang last 30 seconds, and then run to the Ibis at Terminal 3 to drop my bags, then hightail it into Paris for a free day until after dinner. Wow—no obligations, no compromises. 24hours for yourself. Go enjoy. There’s enough to see and do to fill several months, or you could sit in a cafe reading, writing. Go buy some clothes at Les Halles. Visit the Pompidou. Sit in a tea room eating goodies.
I’ve been in this situation a few times but am usually staying a week longer. Love the free time.

First, what Bets and andrew.reis said. But if you insist on your plan, I second the suggestion of Senlis or suggest Chantilly if you insist on your plan to spend your last night outside of Paris. If you had a rental car, these would be great places but the transit schedules for these towns to CDG are not great for a traveler. The site for the buses in the area is https://www.oise-mobilite.fr You can play around at the site to find the bus route for Senlis or Chantilly to CDG. Ultimately, though, I think it makes more sense to bring your family to the airport and then take the train back to Paris and spend the night there or do what Bets said. I don't think you will save that much money to stay out of town and the ease of transit between Paris and CDG is worth something. I adore my family but one day without compromises and doing whatever I want in Paris would be a dream come true for me.

Bets, thanks, that’s a good point. Still trying to decide where to stay the night before I drop off my son. We are coming either from Switzerland or Strasbourg/Kolmar. One thought is a very late arriving train straight to airport, and stay in a nearby hotel. I would love to come in directly from Strasbourg, but in all, I think that’s too risky for me. So a place near the airport for two nights makes a lot of sense. Drop son at airport, and head into Paris for the day, taking evening train back to airport hotel. Sleep in a bit following morning, and head to the airport from airport hotel. Any favorite cheap but nice airport hotels?

Ibis Hotel or Ibis Styles Hotel, both at Terminal 3. You don’t need a shuttle bus to the terminals but can take the internal rail system, CDG-Val. Ibis has an all you can eat breakfast at a reasonable price. There are other hotels with the Ibis name in other locations near the airport but not at the airport. For those you have to take a shuttle which runs every 15-30 minutes and drops you near the T3 Ibis where you catch the internal train. These shuttles can fill, be late....
One step down and you need a shuttle bus is the Ibis Budget. It’s perfectly adequate if watching your budget, breakfast is less expensive but also food offerings are meager.

Ibis is our go-to place for the night before an early morning flight -- but so early that we don't get breakfast at the hotel, just coffee and croissant after we clear security.

The one time I flew separately I put my then-19 daughter on the Air France bus and spent the day absorbing Monet at the Marmottan. Bliss!