Route 222 is the path from Valor Lakefront to Sunyshore City. It's worth noting that it's also the only location where you can find a wild Electabuzz. The two houses on the beach are not essential to visit, but the one on the left is a Pikachu fan club. One of the Pikachu inside is actually a Poké Kid who will battle the player using two Pikachu of her own! The other house has a Fisherman inside who is looking for a giant Remoraid that is larger than 23.7 inches. If you show him one then he'll give you a Net Ball.
Also on this route is the final Honey tree you'll find in the game. The man at the end of the cliff will give you TM56 (Fling). If you ever use Fling in battle, be aware that usually the user's held item will be lost forever upon use (some places where it's kept include the Battle Frontier and player-versus-player battles). Make sure to battle the Fishermen on the beach, as one of them has a Feebas, which is a hard to spot Pokémon that you will need to see to complete your SinnohPokédex. Once you are done here, head through the gatehouse to reach Sunyshore City.
When you enter Sunyshore City, you'll encounter Flint of the Elite Four, who asks if you can try to cheer up his friend, Sunyshore's Gym Leader Volkner, who has been bored lately from the lack of excitement. You can explore around the city if you want, but before you can challenge the Gym you need to find Volkner at the lighthouse. Also, on the beach in the northern area of town, you can encounter Jasmine, a Gym Leader from Johto. It's most important to talk to her after you beat Volkner, though, because she will then give you HM07 (Waterfall).
In the house on the cliff on the eastern side of town, accessible through Rock Climb, is a man that will give you three new Pokétch apps based on the Nature of your Pokémon that are in your current party.
If you have a Pokémon with a Serious Nature inside your Party, you will get the Calendar App.
If you have a Pokémon with a Naive Nature inside your Party, you will get the Dot Artist App.
If you have a Pokémon with a Quirky Nature inside your Party, you will get the Roulette App.
The right-hand house in the northeastern corner of the city belongs to a woman named Julia. Visit her daily to obtain a Ribbon. The Ribbon you receive is dependent on which day of the week it is. She does not give out a ribbon on the first visit.
The large building in the southeastern corner of the city is the Vista Lighthouse. First, use the Dowsing Machine to find a hidden Zap Plate here. Inside, talk to Volkner and you'll leave him convinced to accept your challenge at the Gym.
At the front of the Gym, you will see Flint, who notes that Volkner seems to be excited to battle you and reminds you to help reignite Volkner's passion for battling before departing for the Pokémon League.
Before challenging, make sure that your team is healed. The highest-leveled Pokémon you'll be facing is Level 50. Volkner specializes in Electric-types, which are weak only to Ground-type moves. Ground-types are also immune to Electric attacks, so use that to your advantage! Grass- and Dragon-types resist Electric attacks, but it's not a good idea to use Water- or Flying-types here.
Being the final Gym in Sinnoh, this one has is the most complex puzzle. This one involves hitting switches on the gear-like platforms to rotate them and make the correct path through. In the first room, hit the green switch on the platform on the right once. Then head for the switch on the left and hit it once to reach the next room.
In the second room, go for the nearby blue switch to the left and hit it three times. You can then cross the drawbridge near the stairs and get to the blue switch in the upper left hand corner. Hit that switch then the nearby green switch to get the drawbridge around the right wall ready so now you can go to the third room.
For the third and final room, hit the nearby red switch once and then the one that is nearby to the left to be able to pass the drawbridge on the left wall. Then follow the path to the green switch and hit it twice then hit the blue switch once and follow the path to use the drawbridge on the right wall and return to the first part of this room. Touch the first red switch you used again and backtrack past the right drawbridge again and you'll find the red switch moved the revolving platforms to where you now can reach Volkner.
While all of Volkner's Pokémon are weak to Ground, some are equipped with moves to cover their weaknesses. Most Ground Pokémon will have no problem with Jolteon, but Raichu's Focus Blast and Signal Beam might be threatening. Unless you can take out Luxray quickly, do not use anything that suffers a dual weakness to Ice, such as Torterra, when facing Luxray. Volkner's star Pokémon Electivire has Fire Punch to cover Grass, Ice, Bug, and Steel. It may defeat one of your Pokémon with Giga Impact, but that prevents Electivire from moving the next turn.
Once you beat Volkner, he will be so glad that he was able to have fun that he gives you the Beacon Badge, which allows you to use Waterfall outside of battle. Now that you have all eight Badges, all of your Pokémon will obey you, regardless of level. He'll also give you TM57 (Charge Beam), a damaging electric type move that has a 70% chance of raising the users Sp. Attack by one stage. Now head to Jasmine again and your rival will appear and tell you that he wants to get stronger. Jasmine, who is touched by this conversation, will give you HM07 (Waterfall). With this new HM, you can now head north to the Pokémon League.
On Route 208, Carbos can now be obtained. Backtrack your way to the area where you were first outside in the summit earlier. (See this section for a refresher on how to get there.) Then, use the eastern entrance to the part of Mt. Coronet 4F below the waterfall. Climb the waterfall and go inside the cave to find the Adamant Orb and the Lustrous Orb (as well as the hidden Stone Plate). You will need both Orbs to catch Dialga and Palkia later on.
If you return to the tunnel, since Team Galactic is gone now, you can go through the door that was previously inaccessible. That will take you to Mt. Coronet's north area, where you can find TM02 (Dragon Claw) (as well as the hidden Nugget on the rock just north of it). On Route 210, the Meadow Plate and Wave Incense can also be obtained. Return to Sunyshore City and head north via the use of Surf to reach Route 223.
Route 223 should be a straightforward water route since all you need are Surf, Waterfall, and maybe an Electric- and/or Grass-type Pokémon. You can catch a Mantyke here, but remember that you need a Remoraid in your party for it to evolve into Mantine. At the northern end is the Pokémon League.