walking barefoot in the woods, down a beach, etc...
lots of sun on the bare skin.
My favorite author at an old age felt so good after not dying and enlighten that the orange juice fast gave him a brand new heart that he set out to understand the human body and learned how to become very strong in his 70's and he used his own weight and not lifting weights to build his body muscles.
A person's own weight is all they ever need, it weighs enough and never a need to buy metal weights or go to a gym where bacteria grows and shares every disease in a community.