Right before you get into Nimbasa, a business man recruits you to be the new manager of an avenue. This does not really serve much purpose, but some shops in there do have some really nice stuff. It is worth checking some of it out.
Entering Nimbasa City, and the people in front of the giant building should catch your eye. The player character that you did not pick is trying to get into the Battle Subway, but the conductors, Inigo and Emmet, are stopping him/her. Talking to her/him will actually commence a multi battle between the two conductors and Nate/Rosa. They carry a Boldore and a Gurdurr, and Nate/Rosa carries the starter Pokémon's middle stage that you nor Hugh picked (So, if you picked Oshawott, she gets Pignite, and so on and so forth.) After beating them, Nate/Rosa gives you the Vs. Recorder.
Going left is the Pokémon Center and the Battle Institute, and around back behind the sign is a hidden Thunder Stone for a potential Jolteon or Raichu. Going into the house behind the center and up the stairs is a man who will give you a Sun Stone for evolving a Petilil and the like. Going in the next house over and up those steps, talk to an elderly lady with a Pokémon with higher happiness and you will get a Soothe Bell, and item that raises the holder's happiness.
Going right leads you to a Ferris Wheel, and to Elesa's gym, but it is blocked off by Clyde. He recommends going to the old roller coaster gym, in which beating it gets you a Paralyze Heal. Going further right still by a giant Audino blow-up is another Paralyze Heal.
The other buildings and such aren't immediately available, so going left or right gives new options for Pokémon.