After you reach the end of Route 15: Brun Way, you can go south and use Strength to enter Route 16.
Near the northern entrance of Route 16, there is a ruined house where you can access the southern half of the Lost Hotel.
Battle and Catch
In the grass of Route 16, you can find Foongus, Klefki, Phantump, and Pumpkaboo. In the flowers, you can find Weepinbell, Floatzel, Skorupi, Foongus, Klefki, and Phantump. You may encounter hordes of Murkrow, Foongus, and Klefki. If you Surf here, you may find Lombre and Floatzel. When fishing, you can find Poliwag, Poliwhirl, and Basculin.
Use your Dowsing Machine and walk up the stairs of the ruined building near the north entrance of Route 16. You will find a hidden Max Revive in the corner.
South of the ruined house, you can talk to the girl on the hill to have a Sky Battle with her. She is Sky Trainer Clara, and she has a level 38 Emolga and a level 40 Swellow.
Go west from her through the tall grass and use Cut on the prickly tree to the north. Go north through the tall grass to find a Rare Candy.
South of here, you'll find Roller Skater Jet (a girl), who has a level 37 Delcatty and a level 39 Swanna. Nearby, there is Roller Skater Olle (a guy), who has a level 40 Manectric.
If you go west, then north through the tall grass, you will find a sign with Advanced Tips about the Keen Eye ability. You can also talk to Sky Trainer Gavin who is standing on the hill to the south of the sign. he has a level 37 Chimecho, a level 27 Vibrava, and a level 39 Talonflame.
Now go south along the fallen leaves. Pokémon Ranger Lee pops out of a pile of leaves. He has a level 39 Crawdaunt and a level 39 Sandslash.
Continue east and then south along the path of fallen leaves. Hex Maniac Osanna will come out of the tall grass. She has a level 40 Pumpkaboo.
Go east from the Hex Maniac to find a Lum Berry tree.
From there, turn on your Dowsing Machine and go south along the leaf path to find a hidden Big Mushroom on a pile of leaves.
After that, go east through the tall grass and then walk through the pile of leaves at the end to find a Max Potion. Go back through the tall grass.
If you go west along the leaf path, Pokémon Ranger Bjorn pops out of the leaf pile in the corner. He has a level 41 Sliggoo.
Go north from the Red Ranger and go down the stairs. Down here, you encounter Mysterious Sisters Achlys & Eos. They send out a level 40 Alakazam and a level 40 Gardevoir for a double battle.
To the left of the weird sisters, use Strength on the boulder to push it north into the hole.
There is an area to the north past the yellow flowers that you can explore once you have the TM for Waterfall.
For now, follow the path where Fairy Tale Girl Alice is running around. She has a level 37 Klefki and a level 39 Mawile.
Go past her along the stone path until you reach some stairs. Go down the stairs, then continue walking south through some yellow flowers. Push the boulder with Strength, then go up the stairs and get the Fist Plate.
Turn on your Dowsing Machine to find a hidden Rare Candy on the rock near where you found the Fist Plate.
Now go north through the flowers. Go into the house near the stairs and talk to the fishermen. One of them gives you the Super Rod, which is the best fishing rod available. If you registered the Good Rod, you might want to go into your key items, deregister the Good Rod, then register the Super Rod. The guy who gave you the Super Rod talks about fishing streaks. For more information, read the chain fishing guide.
Go north on the pier next to the house. Go to the right, then use your Dowsing Machine to find a hidden Repel near the fisherman on the square platform.
You can talk to the fisherman on the square platform to battle him. He is Fisherman Seward, who has a level 40 Whiscash.
Go west from there, then south, and talk to the fisherman. This is Fisherman Wade. He has a level 37 Poliwhirl and a level 39 Poliwhirl.
Go north from there, then east to find Fisherman Finn. Both of his pokémon are level 38 Basculin.
Go as far north as you can on the wooden walkways. You will cross the river and reach some land to the north. Go west to find a Dive Ball in the corner.
You have now explored everything in Route 16, so you can now go west to Lumiose City or go east again to reach Route 15 and continue on to Dendemille Town from there.