Step to the East from here along the water and you can enter a small house where you’ll receive a Shiny Stone from the kid inside.  This item is a newer evolution item, and applies to a variety of pokemon- including Minccino, which turns into Cinccino with the use of a Shiny Stone.  The middle person will let you rest your pokemon, healing them as effectively as a Pokemon Center would.

Exit the house and ride the river to the East and you can pick up TM 56 Fling from behind the Deerling house.  Continue along the water and you can reach the dark place known as Mistralton Cave.  If you want to benefit from being here, you’re really going to need a pokemon with you that knows Flash, so it can use the move outside of battle to light up the surroundings.  If you haven’t got a pokemon with Flash (and you probably don’t, as it’s a pretty low-quality move in and of itself), pop back to the Driftveil Pokemon Center to pick up something that can learn Flash and shuffle it onto your team for now.

From the entrance of Mistralton Cave, head South to find a Strength rock.  Get in behind it to the left and shove it right two paces, then down two so that it falls into the hole and lets you continue onward.

Continue South and down some stairs, then head East to run across a hiker.

Battle: Hiker Shelby (1024 Poke)
Opponent: Boldore Lv 32

Past Shelby, spot another Strength rock and shove it East two paces into its appropriate hole so you can get past.  There’s a man here in front of a hole in the wall, but he won’t move aside, only saying that he heard there was a legendary pokemon in here somewhere.  Turn North and climb the stairs, continuing in that direction until you spot some stairs down, which lead to a Dusk Ball.  This item is useful in caves, as it has a bonus to catch pokemon in the dark (whether that’s from a cave or nighttime) or pokemon that are Dark-type themselves.