I actually faced similar problems so I can understand. Skin elasticity keeps going as your age increases but I’m hoping you still have good skin elasticity.
Both yoga and gym are good options depending on what is more suitable to you. That’s the most important step to skin tightening and firmness.
Apart from that you need to have food which is good on fibre. All vegetables, fruits have a good amount of fibre. You also need to scrub your skin well. You should also add healthy omega fats in your body. Fish is a good source. If you are vegetarian than soy, almonds, walnuts and flax seeds. You should also take
vitamin e foods such as almonds to keep your skin healthy.
Also as a last small measure, use a good body scrub while bathing.
Hope this helps.
Thanks
Anant