Sinnoh Route 206
This route lies between Eterna City and Route 207. This route contains a Cycling Road and a grassy area below it. There is also a cave called Wayward Cave found at the northern part of the grassy patch.
Map of Route 206
After you got yourself a bicycle, come to this road and ride on your bike! The slope of the track will pull your bike down towards the bottom. There are a few trainers at this route so be sure to battle them to earn precious experience and cash!
After you reach the southern entrance of the Cycling Route, be sure that you talk to the girl and she will gives you a Flag, a fashion accessory for contests.
When you are moving down the slope, you will be able to see a cave at the bottom of the track. That's Wayward Cave, a place only the wild Pokémon, Gible, can be caught. To get there go to the end of the path under the bridge, go to the left side of the area, take two steps to the right under the bridge and go up. You'll need Strength and optionally Flash.