A guided forest trek to spend a strictly limited hour with a habituated mountain gorilla family, in Bwindi (Uganda) or Volcanoes National Park (Rwanda). Trek length varies considerably by which family you're assigned and where they've moved that morning — anywhere from under an hour to most of a day.
Because this happens outside Tanzania, it's arranged as an extension to a Tanzania itinerary rather than a stand-alone add-on, and requires a separate gorilla trekking permit booked well in advance — permits are limited and often sell out months ahead, especially in Rwanda's Volcanoes National Park.