After you pass through Ula'ula Meadow, you reach Route 17 on your way to Po Town.
Explore Route 17
If you go east from the Po Town Police Station, you find a slope up to a rocky area with some grass. Southwest of the grass, there is a hidden X Attack.
Go north up the easternmost path and you will find a Lemonade to the left.
Keep going north, then go north through the grass at the top to find a Full Heal.
Go southwest and go down the one-way ledge. If you go down the right-hand ledge, you find a Nest Ball.
From here, if you go down the ledge on the left, you can battle a Team Skull Grunt who has a level 36 Mareanie and a level 36 Grimer. You get 1152 Pokédollars for winning.
Go down the ledges from there (you will see a TM on the left, but you have to go back to the top to get it), and if you go south you can battle a Team Skull Grunt who has a level 36 Rattata and a level 36 Haunter. You get 1152 Pokédollars for winning.
Go east to go back up the ramp to the rocky area. Go to the top, and this time go down the leftmost ledges. This will lead you to TM80 Rock Slide.
Go down the ledges, then go west, then north. You can battle a Team Skull Grunt who has a level 36 Ekans. You get 1152 Pokédollars for winning.
To the right of the door to Po Town, there is a hidden Dire Hit on the lamp post.
Go up to the grunts at the door of Po Town and you have to battle them in a double battle. The Team Skull Grunts have a level 36 Pawniard and a level 36 Scraggy. You get 2304 Pokédollars for winning.
After a cutscene, you enter Po Town.