Display discount ad when people visit the page
You are looking for Splash Offer module. It will show the popup on page load.


  Allows the management of one or more "splash offers", which are modal
  popups with an accept or reject form. If a user accepts the offer then
  a new page will open with a provided accept url. If they reject the
  offer, the modal will close. (If they select Do not Show Again, then a
  cookie is set so as to avoid further display.) You may define the who,
  when and where of the splash offers with precision controls. Factors
  such as: user role, site url, device, and whether the offer has been
  previously viewed (cookies), cookie duration, etc. are configurable
  for each splash offer created.


also take a look at this modules - Splashify, Splash and TinyBox (Simple Splash).