Opduin has next to the swimming pool a modest Finnish sauna and Turkish steam bath. This is open for hotel guests from 4:30 pm to 7 pm, you can just walk in with your room key. In the public sauna it is common to be undressed.
Outside these opening hours you can rent the sauna for a private hour. This costs 9,50 per person for 1 hour, which you can also reserve as a non-hotel guest.