If you pick up your car at Blagnac airport you will be northwest of Toulouse, the opposite of your destination and you will have to reach the Toulouse ring road. The whole area between Blagnac Airport and the ring road is quite busy (there are Airbus factories and all the aeronautical industries). And the ring road is busy too
On a Monday, you can therefore add at least half an hour to the travel time indicated by Viamichelin.
The A61 and A9 option will take you to Saint-Rémy-de-Provence probably before 5 p.m.
It's the fastest, the most expensive, and the most... boring.
In case you did not see it during a daytrip, along the way you will see the medieval city of Carcassonne from the A61 motorway
That being said, taking these highways, Bets' idea of stopping at Sète is very good although it could be too late to have lunch there.
Try anyway to go to the restaurants on the quays of the port, there are some interesting ones.
There are other options by taking small roads and going through the Montagne Noire and the Regional Natural Park of Haut-Languedoc with beautiful scenic roads but I'm afraid you won't have time for these because it's is at least 90 minutes longer.
And without a nice weather it's less pleasant
https://goo.gl/maps/Gy1vuLPv26TdCbL8A