Jogging
Jogging is a very cheap way of keeping fit, because all that is needed is a good pair of running shoes.  One can jog anywhere, although it is good to stay away from main roads where the air is full of traffic fumes.  It is also good to avoid too much jogging on hard pavements (sidewalks). A softer surface such as grass is better for jogging because there is less hard jolting for the knees.