Before rushing to pummel the Elite Four and become the Pokemon Champion, there are some optional - mostly brief - areas that you can now access with the Waterfall HM . You can find some nice items here too. If you're taking this little detour, be sure you've got Pokemon that know Surf and Fly too.
Route 15 and 16
Coming from Lumiose City, you'll see the Fishing Shack as usual, right? Go up the stairs to its east, then northeast - using Cut to mow down the bushes - from their apex onto the water. Surf along the river to the waterfall; power up it, then go along the river to the end.
Open up the yellow Pokéball at the end to find TM97 - it teaches Dark Pulse, a decent Dark-type move. That's actually it for here.
In Couriway Town, head to the southern portion of town where the hotel and the Photo Spot are. Ascend the stairs and to the south is a river you can reach. Surf on it, then ascend the Waterfall to the east. At the top, you'll find TM80 - it teaches Rock Slide, a pretty decent Rock-type move. And, yes, that's it for here too.
Route 22 And The Chamber of Emptiness
Fly to Santalune City and exit the city off to the northeast. Once on Route 22, you won't find much to do initially if you've already done everything from the previous visit way-back-when. If you didn't come visit when you got Cut, go north of the Route entrance to Cut down a tree to find an Elixir .
Otherwise, go east to the Victory Road gatehouse; instead of entering, though, go south. Get on the water with Surf, then continue along to and down the Waterfall. At the bottom, go east and onto the land there. Head east and battle the Trainer, then go into the cavern.
Inside the cavern, there is naught but a Spooky Plate , something to raise the power of Ghost moves and to turn Arceus into a Ghost-type. Back outside, head southwest and across the stepping stones. On the other side, turn on the Dowsing Machine as you go south to find a Full Restore in a rock.
Go south and beat the Trainer, then head southeast to the narrow path. Go further east for a while to find a Tanga Berry tree - they can weaken super-effective Bug-type attacks. Now, for a Strength boulder puzzle...
Move the one just to the north south as far as possible, west twice, north once, west twice, north once, and west once into the hole. If you face west from the southwest corner of the boulder once upon it, you can press A to find a hidden Max Elixir . Next, go northwest.
Your immediate instinct is wrong here - push the south boulder westward as far as possible. Go the long way around to its south side and go north once. Head now to the west-facing face and push it east as far possible, then north into the hole.
For the final boulder, push it east five times, then go the long way around to its south side and push it into the hole. There, you will find the useful TM26 - it teaches Earthquake! Go onto the water to the northwest, then head east and up the Waterfall.
Go along the path at the top and you can get a Draco Plate - obviously, it boosts the power of Dragon-type moves and makes Arceus a Dragon-type.
When you return here from Route 20 head to the far west side of the area. Surf onto the water and head north past the music-note-shaped flower patch to the Waterfall. Ascend and you'll find TM29 at the top - it teaches Psychic, a decent Psychic-type move. Also note the guy blocking the cave entrance until you've beaten the Elite Four...
The area in question is found as you enter the Frost Cavern area coming from Dendemille Town. As you reach the snowy area, go northwest past the Hiker and Surf onto the water, then use Waterfall to descend. Once you hit the lower area, go northeast and land to find a Heart Scale .
Go back south and head east along the river. At the end, where some rocks block you, head south and onto land. Grab the TM from the Pokéball - it is TM71 , which teaches the useful Stone Edge move.