I will be in Barcelona for 6 days then we are going to drive to Madrid, but we have one day and night we would like to spend in one city on the coast on the way toward Madrid and yes I know it is not a straight shot. We like older cities with charm and history. Thanks. paul

The usual route from Barcelona to Madrid is the inland motorway via Zaragoza but you can make also the route Barcelona-Valencia-Madrid. Valencia is a big city but is a worth. If you prefer a nice historical village visit Peñiscola in between Barcelona and Valencia.
http://www.peniscola.es/?idioma=en

The coast route to Madrid, via Valencia is about 60 Km longer than the direct one via Zaragoza, both are good motorways with double or triple lane all the way through.
Unfortunately the coast road does not have many older cities with charm.
It is more like , new cities with big buildings. One that could match your interest is Tarragona, but just one hour away from Barcelona.
Probably your best stop will be Valencia , staying in the old part of town.

Thank you both very much. Actually on the way back from madrid I wasplanning on driving back throught the center of Spain thru Zaragoza. Is there something special to see there on my way thru. thanks again. paul

Siguenza, in a short detour, is worth: cathedral, castle, medieval square...
http://www.siguenza.es/siguenza/en/index.html