Almost immediately, a hiker will walk down and give you HM6, Rock Smash. This HM is not currently usable outside of battle, but that will be resolved after you beat Roark, Oreburgh's Gym Leader. To get to Oreburgh, just go to the right, passing behind the trainers' backs if you like.
These items can be obtained by using Rock Smash: Stardust and TM70. To get TM70, go down the stairs from the entrance. Go right, use Rock Smash, and the TM is just to the right. To get the Stardust, go to the left from the bottom of the stairs. Rock Smash past three rocks, and the Stardust will be above you and just to the right on a small ledge.
This item can be obtained by using Rock Smash and the Bicycle: TM31. To get this TM, go left from the bottom of the stairs. Rock Smash past the three rocks, and use the bike (in fourth gear) to jump the ramps. The TM will be just above you.
These items can only be obtained by using Surf and Strength. Surf to the left along the lake. You will reach a ledge with a Veteran trainer on it. Go past the Veteran, then use Strength to push the boulder out of the way to receive a TM01 and an Earth Plate.