I finished black 2 a while ago now I'm looking for something fun to do on it. PS. I've done the black tower already
Well you can always go on a legendary hunt and catch cresselia, latios, kyrum, etc which are all in the game. You can make a team and take on the battle subway challenge, but be warned all those Pokemon are EV trained so they are not to be underestimated. Also, you can take on the the challenges at black/white city then defeat the boss or go to the Pokemon world tournament and take on all sorts of gym leaders and other big trainers (they are also EV trained so be prepared). You can battle the elite four a second time cause they will have higher level Pokemon and pose a bigger challenge. You also have all the online options available like battles, trades, and entralink missions. Beating the elite four is just half of what this great game has to offer. IGN didn't give it a 9.6 for nothing. Good luck!
Hope I helped!
Here are some suggestions:
You could battle at the PWT at Driftveil City to earn battle points.
You could challenge the Battle Subway at Nimbasa City.
You can challenge Alder at his house at Floccesy Town.
You can challenge Cynthia at Undella Town.
You can challenge Hugh and get HM Dive and explore the Abysal Ruins.
You can go to Twist Mountain where you can get fossils and lot of training.
You can go to Pinwheel Forest and complete the task there with Cheren,
You can go to Nacrene City and get the Fossil Pokemon and train them.
You can complete the Pokedex and get the ticket from Professor Junipur to go to the Nature Sanctuary and get a Shiny Haxorus .
You can breed Pokemon and EV train them and use them for WiFi battles.
Hope I helped.
black tower,legendaries,train ticket, egg charm, shiny charm...also what they said.
hope this helps:)