Groningen is an active province and a perfect place to relax in one of the various wellness centres. See some of the venues for complete relaxation here.
At a quiet location in the village of Zevenhuizen is Thermen de Waterlelie. The centre has various saunas, a beauty parlour and a restaurant. Groningen also has one of the few float centres in the Netherlands. Floating is being discovered by a growing number of people; it is ultimately relaxing and beneficial for various problems, from troublesome skin to jetlag.
Relaxing is even better after a workout, and Euroborg sports centre has options for both. If you do not feel like working out but want some relaxation, simply take a massage!
Bad Nieuweschans is known as a spa; an extensive wellness centre in the friendly village offers anything you could possibly expect from a relaxed day.
This is just a small selection of what's available; the choice in the area of wellness in Groningen is huge.