Pokemon Crystal
Annons/Unowns
In the back of each puzzle room in the ruins of Alph is a word formed by Annons/Unowns. They require you to do things in order to get into a back room. In one place, you must equip a Water Stone. In another, you must put a Houou/Ho-Oh first in your lineup. In another you must use an Escape Rope. In another you must use Flash. Once in the back room, pick up the items and fall down the hole to find a room with a message about Annon/Unown. Fall down the hole there to return to the Annon/Unown shrine.
Serebii/Celebi
Hook up the mobile adapter. Go to the communication center in Goldenrod City, and read the message on the lockers to the right of the room. They will talk about a present from the future. Leave and the blue-haired nurse will ask you to defeat all sixteen gyms in order to obtain the GS Ball. Once you have done so, give the GS Ball to Kurt. The next day, he will give it back. Put it in the shrine in the Ilex forest and a level 30 Serebii/Celebi will attack you.
Suikun/Suicune
Find a Suikun/Suicune in three different places (right out of National Park, below the bush by Mount Mortar, and up in Cianwood). Then, go to Tin Tower, and a Suikun/Suicune will appear to fight you.
Mysterious Channel 13.5
Use your radio in the ruins of Alph to tune into Channel 13.5. You will hear mysterious sounds made by the Annon/Unown.