In this case, your best bet would be to advertise in a newspaper read by commuters, such as the Metro. Tailoring the advert to commuters is quite simple, targeting people travelling to work by enticing them with a cheap, convenient city break which they won't have to book time off work for. These types of holidays aren't as effective when advertised on the weekends or in holiday times (such as school half terms for families) as the urgency has gone.