Thermaalbad Arcen is not sightseeing in the usual sense, but it is part of the village's identity as a slow Limburg stop. The baths use warm mineral water and are open to day visitors, so you do not need to be staying at the nearby holiday park.
It works best as a soft landing after walking or cycling in the Maasduinen. If your trip is built around museums and architecture, it is easy to skip.