Looking for a nice one night stop over between Dijon and Nice by car. Trip is in August, possibly near a lake?
It could be a one-day drive (6 to 7 hours by Autoroute) and I'm not clear on the lake focus, but if you wanted to split the drive into two days, you could drive from Dijon to Aix-les-Bains (the word "aix" is a clue that there are, or were, thermal springs there that people used for bathing; hence the word "bain" or bath). Via Michelin plots that out at around 3 to 3.5 hours.
Although I haven't been there, Aix-les-Bains is an old resort town and I would suspect it would have some pretty good sights to occupy a visitor for at least an afternoon and evening.
The next day, you could continue on to Nice. Via Michelin thinks that will be about a 6 hour drive.
That's one option.
Another long diversion would be to go near Moustiers St. Marie/Lac de St. Croix, near the Gorge du Verdon. This involves the route through Grenoble and Gap.
However, instead of a lake, try staying in Isle Sur La Sorgue with the canals and take the slower route through the Luberon before heading on to Nice. Another possible alternative is to stay along the Rhone in Vienne.
Certain routes can get a bit of traffic, so plan accordingly.