OK, I have just copied and pasted from Bulbapedia.
Route 23 is a long mountain road that stretches off to the northwest. On the east side, the path meanders along the river that lies beneath the cliff face. On the west side, small trails wind through the labyrinthine grove of trees. To the north lies Victory Road, a proving ground for Trainers headed to the Pokémon League.
North of the cave exit is a house with a female Ace Trainer inside. Talk to her to have your Pokémon restored to full health. This is especially useful immediately after the events that occurred in the Giant Chasm.
Southwest of the rest house is a small tree near the river. Cut it down or surf around it to reach the Heart Scale.
Head north through the river valley to a large stairway. Wade across the river to a second house; the old man inside will give out TM35 (Flamethrower). Climb the stairway, pick up the hidden Rare Candy to the east, then detour across the bridge to the south. Grab TM05 (Roar) across the next bridge, then head south from Black Belt Luke. Check the rock for a hidden Star Piece, then pick up the Iron to the south and cross the next bridge. A second Star Piece is sitting on the ledge here, and there is a Hidden Grotto off to the south. Once you have collected all the items, return to the top of the large stairway and head west.
A short way down the path is Pokémon Ranger Lena, who gives you a Sitrus Berry when you defeat her. Head west, chop down the tree, and continue on to reach a raised platform to the southwest. Pick up the Full Heal and move the boulder south into the hole. Backtrack a bit and head west to reach Hiker Doug; cut down the nearby tree and move the second boulder southward. Return to the east side of the forest, and chop down the tree here. Head northeast to find an Ultra Ball hidden in the trees, then go west to reach a Repeat Ball. Wander westward through the forest to have Black Belt Benjamin challenge you, which clears the path he was blocking. Cross the platform in the northwest to get an HP Up, then loop around to take the path where Benjamin was standing. Chop down another tree and head south to find a Calcium, then jump either ledge to the east.
Go east and move the third boulder one space to the left. Surf across the water and move the fourth boulder into the hole for easier access. Jump the ledge and push the boulder all the way to the north for later. The fifth boulder sits to the northeast; push it one space to the right so it lines up with the hole to the south. Now go south then west to find a Yellow Shard hidden in a rock. Surf west, then go back east and push the third boulder two spaces right, then one space up to the hole. This gives you an opening to move the final three boulders into place. Shove the second boulder east, the fifth boulder south, and the sixth boulder east.
Climb the stairway and go east to get a Max Potion. Continue east, then go north over the bridge. Inspect the grass patch to reveal a hidden PP Up, then grab TM12 (Taunt) to the west. Climb back down the stairway and go north. Ignore the first "item", and grab the second one, a Green Shard. Jump the ledge and head north towards Victory Road.
Yes, that is for everything you can do in Route 23, but follow it to the end, and you will see how to get to Victory Road.
Hope that helps.