Since Sainte-Marie-de-la-Mer is located right in the middle of the national park, you can see flamingos and white horses. And the park itself if you rent a bike, read more in this article.
There is an arena for bullfighting in the town. It is relatively new, built in 1931, but is listed as a historical monument.
The town is beautiful and convenient, it makes sense to come here with an overnight stay. We did so, slept at the hotel Thalacap Camargue (booking.com).
The embankment is very well done and is the best place in the town. Be sure to have a walk in the evening or in the morning.
The famous painter Van Gogh was in the town and painted a series of paintings.
Getting there: by train to Arles, then by bus 20 to Saintes-Maries-de-la-Mer (map and timetable).