You can battle Cheren evryday day at Victory Road.
You can battle Cynthia at the Villa in Undella Town everyday, from spring to fall.
You can battle the Gamefreak guy everyday.
You can go to the stadium everyday.
You can go on the royal unova ship everday in the evening.
Rematch the E-4.
Those are all the places left in the game that I can think of to train.