There's an Ace Trainer near the entrance who uses a very strong Purugly L68, a Lapras L68, and an Espeon L68. Ouch! Those are some very high levels. Avoid her if you don't want to battle her, because she's definitely a challenge.
Head down the steps and then north to find some thick grass waiting for you. You can surf across the water up there to find a Reaper Cloth, an item that, when held by a Dusclops during a trade, will evolve that Dusclops into Dusknoir.
If you have Waterfall handy, use it to scale the waterfall to the left. Up there, you'll find another of the Team Plasma sages, Giallo! Talk to him and he'll say some stuff before giving you TM08 (Bulk Up). Looker will show up (how did he get here?) and will arrest that guy. Justice!
Just off to the west is a Hiker with a Hippopotas L63 that will brew a sandstorm right away, a Piloswine L63, and a Probopass L63. Beat him, then drop down the waterfall. Heading to the west will lead you to the Abundant Shrine. You can check that out now or come back to it later, but it's recommended you go there with both Tornadus and Thundurus in your party (you'll need to trade with your version's opposite to get the other one).
But since there's still more to check out on the route, head back east towards another tough Cool Trainer with an Absol L68, a Dodrio L68, and a Walrein L68. Beat him and then jump down the ledge to reach near the beginning of the route where you were.
Now head down those steps and take on the Backpacker with her Numel L65. Just north of her is a Black Belt with a Primeape L64, a Scrafty L64, and a Poliwrath L64. Beat him, cross the bridge, but then use Waterfall to climb up the waterfall by the next bridge. There's an Ultra Ball up there in the corner, but other than that, you've seen just about everything on the Route, so head on to the northeastern corner and then make your way to Undella Town.