I've told my children "Saying that you're bored tells me that you have nothing to do." As something can always be found for them to do, they no longer complain to me of being bored.
That was always my parents' solution - when we lived on a farm, bored children got to clean the barn or the chicken coop. When they bought a restaurant and moved in town, bored children got to clean the basement. Or worse, their rooms! :-)