Head south, push the boulder into the hole, but grab the Ultra Ball hidden in the nearby rock first. There's also a Hyper Potion hiding in a rock on the other side of the hole on your way west. Climb the steps, head east, then head south and around the southern perimeter until you reach a Hard Stone, an item that powers up the bearer's Rock-type attacks by 20%.
Now head north and push that next boulder into place, but before you head across it, swing north some more and keep going around the northern perimeter of the cave to find TM80 (Rock Slide), a great, staple Rock-type attack with 80 power that hits for Physical-based damage. It has a good chance of inflicting flinch, plus it also hits two opponents in a double battle. It's a very handy TM to have at your disposal!
You can also slither on through the narrow chasm on the floor, heading around and picking up the Iron, along with a PP up hidden in the western side of the room.
There is a ledge you can jump down if you're feeling gutsy, but you should just go back to that boulder now. Go ahead and push its brother into place, too, so you'll have better access for later. Then, head up the stairs, leading you into somewhere new!