After the Final Gym Badge
* Head to Indigo Plateau: You'll need to travel to the Indigo Plateau, the location of the Elite Four and the Champion.
* Challenge the Elite Four: This is the most challenging part of the game! You'll battle four trainers in a row before facing the Champion.
* Defeat the Champion: After conquering the Elite Four, you'll face the Champion, Lance. Beating him will make you the Pokémon Champion!
Post-Game Activities:
* Post-Game Challenges: There are tons of things to do after becoming Champion!
* Challenge the Elite Four again: You can challenge them as many times as you want!
* Train your Pokémon: With the post-game trainers, you can level up your Pokémon to maximum level (100).
* Catch new Pokémon: There are some rare Pokémon that only appear after you become champion.
* Unlock new areas: Some areas will become accessible after you beat the game.
* Complete the Pokédex: This is the ultimate challenge – try to catch all 151 Pokémon!
Specific to Crystal:
* The Lake of Rage: You can find the Legendary Pokémon Lugia in the Lake of Rage. You need to complete a specific quest and collect the "Goldenrod Radio Tower Key" to access this area.
* The Ruins of Alph: You can find the Legendary Pokémon Ho-Oh in the Ruins of Alph, which can be accessed after you become Champion.
* Suicune & Entei: You might have encountered these two Legendary Pokémon during the main story. If you didn't catch them, you can find them wandering around Johto after you beat the game.
Let me know if you need any more specific help or want to know more about any of these activities!