Here's what you can do after you've obtained the gems and beaten the Elite Four:
Post-Game Activities:
* Complete the Pokédex: There are still plenty of Pokémon to catch! This is a popular goal for many players.
* Battle the Champion again: You can always challenge Cynthia again for another tough fight.
* Catch all Legendary Pokémon: There are several legendary Pokémon to find and catch, including Dialga, Palkia, Giratina, and more.
* Explore the Grand Underground: This sprawling underground area offers tons of opportunities for digging, finding rare Pokémon, and encountering other trainers.
* Train your Pokémon: Now that you've beaten the game, you can focus on building powerful teams and battling trainers online.
* Try different strategies: Experiment with different Pokémon teams, battle styles, and item combinations to find what works best for you.
* Explore the game's secrets: There are many hidden secrets and Easter eggs to discover in Pokémon Brilliant Diamond and Shining Pearl.
* Train for the Battle Tower: This post-game challenge is a great way to test your skills against increasingly difficult trainers.
* Complete side quests: There are various smaller quests to complete around the game world, including the "Secret Bases" feature.
* Play the game in a new way: You can try a "Nuzlocke" challenge, a "No-Item" run, or any other interesting variation to make the game more challenging and fun.
Other Options:
* Play the game again on a higher difficulty: If you want a greater challenge, you can replay the game on a higher difficulty level.
* Trade Pokémon with friends: Trading with other players is a fun way to get new Pokémon and build unique teams.
* Try other Pokémon games: There are many other great Pokémon games available, each with its own unique story and gameplay.
Remember, the fun doesn't have to end after you beat the game. There's still plenty to explore and experience in the world of Pokémon. Choose what interests you most and dive in!