At first, this paper seemed to me to be contradictory: having defined VILPA as excluding exercise for the purpose of exercising, it recommends doing it for that purpose... On reflection, I think the advice is directed towards people who could take the stairs, but habitually don't. Aside from that and running to the next bus/train stop, I'm not sure there are all that many options for this sort of behavior modification without setting aside some time for it.