Walk north to the tree stump to find a Big Mushroom. Keep walking north and you'll run into a backpacker that says there is a woman who prefers to be alone in the camper. If you have brought an event Shiny Raikou, Entei, or Suicune over from a Generation IV game with the Relocator, she will show her true form as you try to enter the camper. She is actually a Zoroark that you can catch.
Special Pokemon in Lostlorn Forest:
Zoroark (Bring an event Shiny Raikou, Entei, or Suicune to the camper.)
Beyond that, there really isn't anything to do here until you get the HMs for Surf and Waterfall. Go back to Nimbasa City and take the northwestern exit to Route 5.