Will be spending a few days in San Sebastian in late August. Have hired a car for other purposes and am considering a day trip, possibly overnight, to see Biarritz and some of the French Basque countryside.

What border cross difficulties should someone traveling on an American passport expect? I presume I'll need to coordinate with rental car agency to obtain whatever tax stickers might be required. Considering the time of year and recent reports of unrest should I be expecting absurd queues?