Hatch a Shiny Pokemon from an Egg. (Not Ditto)

Obtain your first gym badge.

Obtain your second gym badge.

Obtain your third gym badge.

Obtain your fourth gym badge.

Obtain your fifth gym badge.

Obtain your sixth gym badge.

Obtain your seventh gym badge.

Obtain your eighth gym badge.

Obtain your ninth gym badge.

Obtain your tenth gym badge.

Obtain your eleventh gym badge.

Obtain your twelfth gym badge

Obtain your thirteenth gym badge.

Obtain your fourteenth gym badge

Obtain your fifteenth gym badge

Obtain your sixteenth and final gym badge.

Rematch Falkner on Set 16.

Rematch Bugsy on Set 16.

Rematch Whitney on Set 16.

Rematch Morty on Set 16.

Rematch Chuck on Set 16.

Rematch Jasmine on Set 16.

Rematch Pryce on Set 16.

Rematch Clair on Set 16.

Rematch Brock on Set 16.

Rematch Misty on Set 16.

Rematch Lt. Surge on Set 16.

Rematch Erika on Set 16.

Rematch Sabrina on Set 16.

Rematch Janine on Set 16.

Rematch Blaine on Set 16.

Rematch Blue on Set 16.

Why doesn't every Pokemon game have this?

Change yourself. (After having started the game)

Defeat ??? in Slowpoke Well.

Defeat Youngster Joey after getting all 16 Badges.

Sit through ShockSlayer's optional intro speech. (It's a lot of text)

Find something under the truck. (Without having seen data before)

Walk through the border along Route 27 for the first time.

Obtain the Old Amber without asking for it.

Obtain the DV Viewer upgrade.

Catch all 251 Pokemon. Yes, all of them.

Defeat all Battle Sim trainers. (Don't leave building, use Center, or deposit_withdraw on PC)

Enter the Hall of Fame for the first time.

Enter the Hall of Fame again.

Get the Red Scale at the Lake of Rage with 3 or less badges.

Find a picture of the Pokemon so rare, it's not in the dex.

Have SS forge you a sword.

Find and defeat Yellow in her secret fishing spot.

Defeat ??? at Mt. Silver.

Teach a Pokemon in slot 1 Pay Day from the Gen 1 Move Tutor in Kanto.

Teach Surf to a Pikachu in slot 1.

Reach Round 7 in Voltorb Flip. (Goldenrod)

Win in the slots three times by betting one coin. (Don't leave machine) (Celadon)

Catch every variant of Unown.

Teach an Unown a second move.

Tune in to a very spooky signal at the Ruins of Alph.

Max out your Blue Card and get Buena's Number.

Hatch a Shiny Ditto from an Egg.

You've beaten the game entirely.

Catch Articuno, Zapdos, and Moltres.

Catch Raikou, Entei, and Suicune

Catch Mewtwo.

Catch Ho-Oh and Lugia.

Catch Mew and Celebi.

Catch Pigy. (missable info in comments)

Sit on a bench.

Defeat all Gyms (R16) and then E4 (Rematch) with one Pokemon. [1 session | no dying | music not disabled]