battle tower/subway, breeding to get good pokemon and trade or use them for battles ( either battle tower/subway or online), finish the specific post game quests other ppl already mentioned, go hunt legendaries and items, etc.
you could even explore the area searching for easter eggs.
oh and you have pokemon world tournament as well if you're playing black or white 2.
there's usually tons of stuff and ways to keep you busy.
just look around for a bit.