One of your post-game tasks is to search Unova for the Six Sages of Team Plasma who ran away after you defeated Ghetsis. All but one of them will surrender without a fight. Each one will give you a TM. The names of the sages are: Bronius, Giallo, Gorm, Rood, Ryoku, and Zinzolin.
Bronius is hiding out in Chargestone Cave. He's all the way in the basement, down some stairs that are near a female Ace Trainer. When you talk to him, he will have two Team Plasma grunts attack you. The first one has an L63 Watchog and L63 Scrafty. The second one has two L62 Liepards and an L62 Garbodor. Once you defeat the Grunts, Bronius gives you TM69 Rock Polish, and Looker runs up and arrests the Team Plasma guys.
You will need to teach Waterfall to a pokémon. You can get HM05 Waterfall on Route 18, which you reach by surfing west from Route 1. Anyway, use Waterfall to go up the waterfalls in Route 14 until you find Giallo. He will give you TM08 Bulk Up and then Looker will arrest him.
Gorm is in the basement of the Dreamyard, which can be accessed now that you have completed the main game, because the traffic cone that once blocked it is gone now. You will have to fight some trainers in order to get to some stairs that lead to Gorm. He will give you TM75 Swords Dance before Looker takes him away.
If you Surf west from Route 1, keep going west to eventually reach Route 18. Rood is on the beach in the southwestern part of Route 18. Talk to him and he will give you TM32 Double Team.
Go to the bottom floor of Relic Castle and go through the new door that is there. Follow the Grunt that runs away and you must fight his L64 Krookodile. Then continue south, then west, then south, then west twice. Ryoku is there and he will give you TM04 Calm Mind. You can go north to get Claydols and a Volcarona.
Zinzolin went to hide in Cold Storage. In fact, he is in that container that you went into to find the Team Plasma people before. When you go into there, talk to him to get TM01 Hone Claws.