How to remove dullness of mind, without sitting meditation, in regular activity?
You can indeed meditate(Vipassana) while performing any task. 
ex: if you are sweeping, keep reminding: sweeping.. sweeping..  sweeping..

Dullness or boredom is a form of aversion. Ignorance is the cause of it. 
To overcome dullness, simply note it as it is without taking it as "I am bored" or "it is boring".