Well, there is a TON of stuff to do.
Find the 7 sages.
Explore the 3 new routes to the east of Unova.
Check out Black City and White Forest.
Rematch the Elite Four, this time with the actual Champion.
Catch Cobalion, Terrakion, Virizion, Kyurem, and the roaming legendary.
Catch Zen Mode Darmanitan.
Catch Musharna with the Dream World Ability.
Recieve the fossils in Twist Mountain.
Buy a Magikarp.
Recieve the Larvesta egg.
Trade for Rotom and Munchlax.
Beat Cynthia and Morimoto.
Battle Cheran and Bianca.
Check out the Abyssal Ruins.
Complete the Battle Subway and Battle Institute.
Take a ride on the magnificant Royal Unova.
Do Pokemon Musicals.
Utilize the Poketransfer.
These are but a few of the many things you can do. It really is quite amazing all the after the main game stuff they have.
The high leveled Pokemon are there for easy training.
Some of this stuff can be done before the Elite 4, but you might not have done them yet.