There is often yoga available on the Wonder, and there is no cost. It depends on how long the cruise is, because I don't think yoga is one of the more popular activities.
On some of the longer cruises (WBTA, EBPC) my DW and I have been on, we've organized yoga practice groups, and held them on deck 7, aft (the "secret" deck), which is amazing, especially early in the day, near sunrise.
All the best,
Richard