Bad Kreuznach is a little town in Rhineland-Palatinate, well known as one of Germany’s best health spa resorts.
Its main asset is the thermal spring water, rich in minerals, pumped up in swimming pools from about 500 meters depth, with a salt content of 1.5-2 %. You can enjoy the benefits of thermal spring water in one of city’s spa centers, like Bäderhaus or Crucenia-Therme (the name resembles Cruciniacum, the old, celtic name of Bad Kreuznach from the 5th century BC).
After the spa session, I can recommend Pizzeria La Perla, a nice little family-owned italian restaurant, where Rosa, Franco and their son, Roberto, will welcome you with good food and one of the best coffees I’ve ever had.
You can also take a walk around the town, they have a nice shopping street called Mannheimer Strasse, with plenty of shops and restaurants.
I should warn you that Bad Kreuznach is peaceful and quiet, so not exactly a party destination.
The Eltz Castle is one hour drive away.