There are several threads on routes from Calais down through France, for sure. Several others on Spain, too. Trip reports and the Travel section will turn them all up.
For example:
http://www.ukgser.com/forums/showthread.php?t=197540
Many of the autoroutes are free on the westside of France.... assuming you don't mind motorways as long as they are free.... The viaMichelin website (or half decent maps) will show them all....
...In fact maps hold much of the answer (and all the fun) of answering your question.... Have a go at plotting your own.....
...Only you know how far you want to go in a day, how long you have got to get to San Sebastian and back to Jockland, where you might want to stay, what sort of roads you really want etc. etc. etc.