The way to get children out of doors is to get out of doors oneself. My DS & DDiL love going for country walks, visiting ancient monuments, and old churches, and now they have children country parks, zoos and other attractions so DGC have spent their childhood going for long country walks and miss them if the weather makes them difficult.
It is like everything from reading to not living as if a screen was surgically attached to the hands with strings to the eyes. If the family, especially parents, do/don't do it the children follow their example