Question: what to do after beating pokemon league sapphire?
Answer: Complete the Hoenn Pokédex.
Obtain all In-Game Gift Pokémon.
Master the Battle Tower.
Complete all Chambers of the Trick House.
Visit Mirage Island and Capture Wynaut and Obtain Leichi Berry?
Master all ranks of the Beauty, Coolness, Smartness, Cuteness and Tough contests.
Collect all Technical Machine (TMs).