Recommended areas to stay north of Paris
 

I will be flying into Beauvais Paris airport (the only flight we could get) and will stay one night to attend an expo at Charles de Gaulle before we drive onto Reims. The rest of my family will have a free day to sightsee. We do not need to stay near CDG as I can get there separately. I was considering Saint Denis so they are close enough to central Paris if they want to visit, however are there any other nice places north of Paris that would be worth seeing?

St Denis is not really a nice place to stay and same for all northern suburbs.
Beauvais airport buses bring you to porte Maillot, which is a much safer place. There is now a brand new train line connecting Porte Maillot to Gare du Nord (Magenta) where you can connect for CDG.

Depending on family interests, Chantilly and Senlis could also be options. By public transportation, it takes between 1.5 and 3 hours to get from the Beauvais airport to Chantilly.

You don't have to go anywhere. You can sightsee in Beauvais which has a majestic cathedral, a departmental museum, a vast grand place, another impressive church, and a site from the Middle Ages (Maladrerie St Lazare) which has a lot of interesting and varied programs. There is also a tapestry manufactury which deploys a technique, basse-lisse, popular in the first part of the 18th century, used now for the creation of contemporary works:

https://www.calameo.com/read/0071833...71642fc?page=1

Thank you everyone for the great recommendations, it was very helpful! We have found lovely accommodation near Chantilly so have decided to stay there.

Saint Denis is perfectly fine and is served by 5 metro stations. Aubervilliers is another good option and it has 6 metro stations. There is such a prejudice about these two working class cities that people who have never been to either place will always warn you away because ignorance is scary.

it seems that people who live there don't agree with you (except for public transport).
https://www.ville-ideale.fr/saint-denis_93066