Lake of Mouriscot is a popular place to enjoy some time away from the hustle and bustle of everyday life. See if any water activities are on offer here, or just enjoy the calm ambiance.
You'll be spoiled for sightseeing choice when staying in Biarritz. Lake of Mouriscot is a good place to kick off here, but there are lots of other interesting places to explore. Cote des Basques, Grand Beach, and Biarritz Aquarium are near the body of water and will round out your day nicely.