Tip number 1: from Breskens, you can book a boat trip with De Festijn shipping company to go in search of seals in the Westerschelde. There are no guarantees, but you almost always see them. And the boat often gets very close. Great fun!
Be sure to book this excursion when the weather is nice, as the boat is mostly open. And make sure you book online in advance at www.defestijn.nl, as the safaris are almost always fully booked. It is therefore rarely possible to just drive to the harbour on the off chance.