At Werribee Zoo in Australia, visitors were encouraged to bring their own torches to spotlight the animals. They were asked to cover the torch with red cellophane so that the animals would not be blinded by yellow or fluorescent light. In addition, there were guides/keepers stationed at each animal enclosure with powerful torches to illuminate the animals if visitors did not have torches. Visitors were also encouraged to shine their torches into the trees to see wild ringtail and brushtail possums, owls, and tawny frogmouths.
For an extra charge, visitors were taken out into the lower savannah on the back of an open-air ute (Yanks would probably call it a 'truck', Poms would probably call it a 'pickup') with a keeper to spotlight the zebras, antelopes, giraffes, rhinos etc.
There were also conservation-oriented comedy plays to get the 'fighting extinction' message across to kids, as well as fire dancers and marshmallow roasting. I think that the idea was that the zoo had to offer something more than the ability to see the same exact animals at night that you would see during the day.
Night zoos are a great idea, I think, to maximise the earnings from the fixed assets at the zoo. Visitors seem to love the idea of bringing out their kids for a 'night out'.