Part 10: Champion — the post-game
Cerulean Cave opens and Mewtwo waits on its bottom floor. Faraway Island, the Battle Tower turned into a shop for the Battle Points you win, Title Matches at the Indigo Plateau, a second team for every gym leader and then a post-game team on top of that, post-game Elite Four rosters, and Team Rocket fugitives to track down once a day. Beat the Champion a second time and you are given the Mega Ring; after that the Mega Stones start appearing around Kanto, with Mewtwonite X and Y on the second floor of Cerulean Cave.
Champion — the post-game opens 6 areas. There are 3 trainers standing in them.
Where to go
Cerulean Cave 1F
Wild: Magikarp 100% · Lv 5, Slowpoke 65% · Lv 30-50, Poliwag 60% · Lv 10, Rhyhorn 50% · Lv 50 +11 more
Pick up: Full Restore, Max Elixir, Nugget, Ultra Ball (hidden)
Cerulean Cave 2F
Wild: Rhyhorn 50% · Lv 50, Golbat 40% · Lv 52-57, Kangaskhan 30% · Lv 54, Chansey 20% · Lv 56 +2 more
Pick up: Full Restore, Mewtwonite X, Mewtwonite Y, PP Up, Ultra Ball
Cerulean Cave B1F
Wild: Magikarp 100% · Lv 5, Poliwag 80% · Lv 10, Slowpoke 65% · Lv 30-50, Rhyhorn 50% · Lv 50 +12 more
Pick up: Max Revive, Ultra Ball
Battle Tower
Trainers: Black Belt Jiraiya Lv 85
Erin's House
Trainers: Cooltrainer Erin Lv 72
Faraway Island
Trainers: Lass Kairi Lv 75