Please confirm what kind of employee engagement activities are in your mind. Secondly, why do you feel that they need engagement? As far as engagement activities are concerned, have you divided these into two parts i.e. physical and mental or intellectual engagement. If these seniors prefer latter, then why push physical engagement activities through their throat?
By the way, if these people are seniors then what is the quality of their supervision? Are they HODs? If yes, then are they meeting their department's targets?