You have to go to the Sevii islands and from there you must retrieve the Ruby and Sapphire for Celio, the Ruby is located on 1 island and the Sapphire is located on 5 island give the Ruby & Sapphire to Celio who is located in One Island. Once done Cerulean Cave will not be guarded.
After becoming champion of Hoenn by defeating the pokemon league
you need to become Pokemon League Champion then help Celio on One Island to get his computer running
beat the Pokemon league
In the Pokemon league.
Pokemon Crystal allows you to travel to Kanto and eventually participate in the Indigo League at the Indigo Plateau, after becoming Johto League Champion.
Your rival is the champion.
You have to beat the original champion in the game
he is the Pokemon league champion
Lance
the Pokemon league and maybe cerulean cave
After you defeat the elite 4 as well as the champion Gary the only new area that opens up is the mystery dungeon in Cerulean city. In this dungeon, you can find rare and high level pokemon.
The legendary pokemon available to you in this version of pokemon are located in these areas: Power Plant, Seafoam Islands, Mt Ember, Cerulean Cave. Also after becoming a pokemon champion at the pokemon league you can find an Entei/Raikou or even Suicune depending on what starter pokemon you chose Entei is available if you picked Bulbasaur and Raikou is available if you picked Squirtle and Suicune is available when you chose Charmander.