We want to take three full days/two nights to drive a rental car from CDG to a conference I have in Madrid starting on the 4th day. Is it feasible to drive the one day to Lascaux in Montignac, France, take the cave tour next day and then take the road through Toulouse to Huaca, Spain in a day? or perhaps better solutions from other travelers? The partner loves to drive and is used to driving 12 hours a day for a living. The real question centers on what road(s) cross the Pyrenees that give great views and are doable in a rental car with standard transmission? We will adjust whether we spend 2 nights in France or one in each country depending upon how long/which way it takes to cross the Pyrenees. The rest of our trip after Madrid will be driving the coastal route to Pisa, Italy, so that is why we don't want to go the coastal routes around the mountains on the way south.
Use viamichelin to calculate your routes: https://www.viamichelin.com/
This gives routes, distances times and costs (fuel + tolls). The times assume you don't stop for anything, so in practice add at least 25%.
Autoroutes /Autopista ("A" road numbers) are the quickest, but also boring. They require tolls in both countries. Routes National in France ("N" route numbers), and the equivalent is Spain are more scenic but slower and go through towns.
Virtually any road over the Pyrenees is scenic. Any road marked on a road map will be OK, this isn’t the Australian outback, all roads are hard surfaced. I like the coast route at the Mediterranean end from Spain to France, follows the coast.
Renting a car for 17 day trip plus two travel days -- from Paris to Madrid to Italy and then back through either France or Austria to Germany and then France.
Good, returning it in another country is expensive. This is a lot of driving!