Not sure about the Wonder but they do offer Yoga classes on the Magic. These were originally offered at no charge but while on my last sailing on the Magic (May 1st, 2004) I noted that they now have a $10.00 per class fee.
In my personal opinion, this new fee should not have been implemented. Are they going to charge for the Fitness Room next? $15.00 for 30 minutes on the treadmill?!