In 1992, Nickelodeon got together with the Kids World Council to decide what was most important to kids at the time. Then they assembled those items, put them in a big orange time capsule, and buried it in front of the Nickelodeon Studios.
The time capsule was removed in August 2006 and was replaced with a concrete slab. It is now buried outside the entrance of Nickelodeon Suites Resort where it is set to be opened in April 30th, 2042. Here's a list of everything inside.