After defeating Elesa in the Nimbasa City Gym, you can go to Route 5. When you step into the route, Cheren will come through the entrance that you just came through, and he will challenge you to a battle. He has a Liepard, his starter, an elemental monkey of the same type as your starter, and a Tranquill.
Once you defeat Cheren, Elesa will come along and introduce you to Alder, who is the current League Champion. Alder asks you and Cheren to have a double battle against a couple of preschoolers. First, Cheren is kind enough to heal up your pokémon for you. Both of the preschoolers have a Herdier. Cheren will use his Liepard.
After defeating the preschoolers, Alder will have more words of wisdom for you. Cheren seems to think that becoming the League Champion is all about becoming the most powerful, but Alder says that becoming the Champion is also about love for pokémon.
After this, you can talk to all of the people here. There are some trainers: the Baker, Musician, Dancer, and Artist all have pokémon that you can battle.
Inside of the camper, there is a woman who will buy food items from you. She always pays a better price than the pokémarts. She will buy things like Big Mushrooms, Lava Cookies, Berries and so on. The price that she pays you for vending machine products is the same as you pay at the vending machines; you don't profit if you sell her those.
When you're ready, go to the west to find Cheren and Elesa waiting at the drawbridge. Elesa will call Clay, who is the Gym Leader of Driftveil City, and ask him to lower the drawbridge. You'll see the drawbridge being lowered, and then Elesa will head back and Cheren will go ahead to Driftveil. Now you're free to cross the bridge.
Crossing Driftveil Drawbridge
There isn't much to do on Driftveil Drawbridge. There are no trainers here. However, be sure to be on the lookout for shadows on the bridge. You will also hear a rustling sound when you see the shadow. If you step on the spot where the shadow is, you will either receive a feather or encounter a wild Ducklett.
Feathers are similar to, but not exactly the same as, vitamins. They add 1 EV (effort value) to your pokémon. However, they can increase your pokémon's EVs to 255 instead of the limit of 100 with vitamins.
As for Ducklett, it is a Water/Flying pokémon, which is a useful type for the upcoming Gym.