The area has special spots where you can use your Acro Bike to hop on and reach the other areas otherwise would be inaccessible. From the topmost stairs, go down two ledges then hop down to the right to find TM43 Flame Charge. Backtrack to the middle path and head west. Go down the cliffs to find a Super Repel.
Now slide down and head west. There should be a few stepping stones and long, narrow slippery slope that can be accessed by any bike. There's a Secret Base entrance here and TM69 Rock Polish.
From your position, heading south will take you to a few more ledges. You'll encounter Hiker Eric there. A bit to the south of where Hiker Eric is, you'll find three slopes. Slide down in the middle to find a Full Heal. Another trainer will be waiting near the edge of the grass patches.
Hiker Eric (Reward: p800)
Fairy Tale Girl Nellie (Reward: p368)
After defeating the two of them, you can head east a bit to find more stepping stones that you can use to hop your Acro Bike and backtrack items and trainers you passed by. Make your way back up using the stones on the eastern side of the area and challenge Expert Shelby. From her position, head to the left and examine the small stone in the middle to get a Full Heal. (use a Dowsing Machine if you want to make sure)
Expert Shelby (Reward: p2,080)
Head a bit to the south to challenge yet another trainer. After defeating him, equip your Dowsing Machine. Search the tree east of his position to find a Great Ball. Next, jump to the ledge to the south to find a Max Ether.
Camper Ethan (Reward: p384)
Continue to the south and fight the next trainer. Once done, head to the west to find stepping stones leading south. Follow that to reach the X Defense at the end. The Mega Stone Tyranitarite is also found in the area but not after you go further in the story where you'll discover Kyogre / Groudon.
Picknicker Diana (Reward: p352)
Head south to reach the small portion of Route 112.