If you are staying in Biarritz then its an easy day trip to the beautiful city of San Sebastian which is often described as one of the world’s top spots for food and has more Michelin stars per capita than anywhere else. The city also has the advantage of having wonderful beaches including the bay of La Concha. With it’s surrounding mountains the city has a touch of Rio about it.
If you are staying in the centre of Biarritz then you can get the train to San Sebastian but you would have to change trains at the boarder resort of Hendaye and the pick up the local Euskotren train. If connections are good the whole journey takes about 1 hour 20 minutes. The disadvantage of taking the train is that the railway station at Biarritz is quite some way out from the centre. You would need to take a taxi or number 10 bus. The bus only costs a euro.
A better alternative would be to take the coach service run by ALSA. From the centre, its an easy walk to the coach stop at Avenue Charles Floquet. Your hotel would be able to give you directions. To save time, tickets can be purchased online at https://www.alsa.es/en. You will need your passport number if you are booking online and current return prices are only 14 euro. Depending on traffic, the journey from Biarritz to San Sebastian takes about an hour.
There are plenty of coaches running throughout the day and the coach station at San Sebastian is an easy walk to the city centre, the old town and the Bay of La Concha. The bus station is located just over the bridge De Maria Christina Zubia. You could get the 9:50 coach from Biarritz and return of the 16:00 coach for example.