The Delta Episode. That is the biggest.
You can also face the E4 again, now they have stronger Pokemon.
There are some optional quests you can also do if you haven't done them yet: http://m.ign.com/wikis/pokemon-omega-ruby-alpha-sapphire/Optional_Quests
There are these too: http://m.ign.com/wikis/pokemon-omega-ruby-alpha-sapphire/Post_Game
