The answer depends on how 'free' the free has to be.
Writing but if you could not afford the pen and paper to begin with it would not be free.
Gardening, but you need to acquire access to the plot of land and to get the first batch of plants growing; even if you collect seed the seeds in future years.
You cannot eat or cook without obtaining the food to begin with.
Things that appear to be free often have a price. Even conversation uses up time which some say is money.
Better end the list there take a walk and get some fresh air, which still is free. Maybe, it is n't because you need shoes to walk in and as its cold a coat to wear.