HIKING is an acronym for Herculean Incitement to Kindle Incredible New Gambits. It is seen in the anime as a harsh training regimen for shinobi students.
In order to improve their survival skills, students must weather extreme conditions for two full days with almost no outdoor gear. Furthermore, they must complete their assigned mission. The assigned mission can most likely vary as the mission is chosen by the one who has organized the HIKING trip. It's one of the most demanding training regimes for shinobi.
Additionally, HIKING trips are done at a specially made THEME PARK to promote further danger and struggle to shinobi students as they proceed in trying to complete their given objective.