This is a spa night for the whole relief society can attend. Set up the gym like a real spa, flowers, scented candles, healthy food around,and everything you could find in a spa. Offer babysitting service also. Have all your Young Women help by giving Pedicures, Manicures, and Helping the ladies soak their feet.
If you want you can make a homemade facial. You could have a couple girls sing a duet or a solo.
Make it a relaxing night for the Relief Society. It helps the girls get to know the relief society better and gives them a chance to enjoy the service project. :)