Remember when that first map of the Unova region was released, back when we were still referring to it by its Japanese name of Isshu? We saw an airport on the map and got super excited thinking of the possibility that we'd be able to fly to previous regions or at least fly somewhere cool on a plane. Mistralton City is home to that airport, although alas, the town's Gym Leader, Skyla, quickly explains that the airplanes are for cargo only. Professor Juniper's dad also stops by to upgrade your Pokedex so that it can display a Pokemon's various forms.
When you're done exploring Mistralton (make sure to check out the Move Deleter and Relearner's house, head straight north on Route 7 until you reach the Celestial Tower.
Wild Pokemon (~levels 26-29):
Ah yes, a Pokemon game would not be complete without a creepy mausoleum full of Pokemon graves and Ghost-type Pokemon. The Celestial Tower isn't spooky like the Pokemon Tower in Lavender Town though – it's more of a peaceful place of respect. Head up through its four floors battling trainers and wild Litwicks and Elgyems – the nurse trainer heal you at about the halfway mark. There are also two TMs to pick up along the way, TM61 Will-O-Wisp and TM65 Shadow Claw.
Once you get to the top and ring the bell, you can Fly off the top of the tower if you don't feel like traipsing all the way back down. Now let's go throw some rocks at Skyla's Flying types. Rollout!
Wild Pokemon (~level 26-29 normal grass, 30-33 doubles grass)
Normal/doubles grass: Deerling (color varies by season), Tranquill (Spring, Summer, Fall only), Cubchoo (Winter only), Watchog, Zebstrika, Foongus
Shaking patches: Audino (common), Emolga, Unfezant (rare, Spring/Summer/Fall only)
This time, head northeast past the Celestial Tower to continue on Route 7.
As you approach the base of Twist Mountain, Cheren challenges you to a battle, and afterwards Alder shows up and gives you HM03 Surf. There's a ton of new areas to backtrack and explore now that you have Surf. You can revisit places you've been before and find newly accessible items and new Pokemon to catch, but there are two major detours worth discussing in detail.
TWO POST-SURF DETOURS (Optional - Skip to next page to continue with the main story)
Detour 1: Legendaries Cobalion and Virizion
Hunting legendaries can be a daunting task, but you might as well get these two out of the way. Cobalion and Virizion are the first two in the legendary trio of Black and White. You need to catch Cobalion before you can encounter the other two, so it's worth it to at least take care of that one before you head to Victory Road, where Terrakion resides.
To find Cobalion, head back to Driftveil City and north onto Route 6. Now that you can surf, you can jump off one of the bridges near the entrance and surf along the river (where you can find YM84 Poison Jab, by the way). Near the house where you can heal, head east into the grass past the rocks blocking the river and then use surf, continuing east until you reach the entrance to Mistralton Cave.
Lots of boulders to move around in here, but if you use TM70 Flash (you can find this in Castelia City from the man behind the dumpster in the alley) you'll see the path is straightforward. Head along the path moving boulders and fighting trainers until you read the steps leading up to the next level. Proceed through that level and up into the Guidance Chamber. An old man is standing to the east – listen to his story about the legendary trio before proceeding. Move the two boulders, and save your game before approaching Cobalion, who appears at level 42, and is Water/Fighting type. Dusk Balls are your best bet.
To catch Virizion, head back to Pinwheel Forest. On the main road just past the bridges, a small gap in the fence has opened on the eastern side. Head east until you reach a clearing, then go through the trees to enter Rumination Field and face Virizion (again, remember to save!).
Virizion is tougher to catch than Cobalion because it has a recovery move, Giga Drain, that makes it difficult to keep its HP low enough to catch. Using Sleep or Paralyze against legendaries is always recommended, but in this case it's particularly helpful because it'll prevent Virizion from recovering.
Detour 2: Route 17, 18 and P2 Laboratory
Note: before setting off on this detour, make sure you have at least one empty space in your party so that you can receive an egg. Leave even more spaces empty if you're planning on catching wild Pokemon too.
If you fly all the way back to Nuvema Town and head north on Route 1, you'll notice some surfable water to the west. Surf southwest a little bit (the only Pokemon in this water is Basculin) and you'll eventually reach the gate to Route 17.
Not much dry land on Route 17, but you can surf around and walk in shallow areas, battling swimmers as you continue west. The water is full of currents that push you along a set path, and you can ride each one to explore the area.
When you're ready to continue, head southwest and you'll see a set of three currents. Ride the southernmost one to reach dry land on Route 18.
Wild Pokemon (~level 28-31 normal grass, 32-35 doubles grass)
Normal/doubles grass: Scraggy, Dwebble (normal grass only), Crustle (doubles grass only), Watchog, Sawk (Black only), Throh (White only)
Shaking patches: Audino (common), Throh (Black only, rare), Sawk (White only, rare)
Bubble spots: Alomomola, Jellicent (rare)
Head up the stairs to the house and a ranger inside will offer you a Larvesta egg if there's room in your party. The woman will also heal your party if you talk to her.
Head down the steps to the west and you'll find HM05 Waterfall at the end of a long, narrow, grassy ridge. Then head north up more steps, east across a bridge, and down to the water to surf again. Surfing along the currents leads you to another part of Route 17, this time on dry land. There's only one thing to check out here - P2 Laboratory.
The only thing inside the lab right now is a Dubious Disc, so go ahead and pick it up and we'll be on our way back to the main quest.
Part one - Nuvema Town, Route 1, Accumula Town, Route 2, Striation City (Tri Badge), Dreamyard
Part two - Nacrene City (Basic Badge), Pinwheel Forest, Castelia City (Insect Badge), Route 4, Desert Resort, Nimbasa City (Jolt Badge), Route 5, Driftveil City (Quake Badge), Cold Storage, Route 6, Chargestone Cave
Part three - Mistralton City (Jet Badge), Celestial Tower, Route 7, Legendaries: Cobalion and Virizion, Route 17, 18 and P2 Lab
Part four - Twist Mountain, Icirrus City (Freeze Badge), Dragonspiral Tower, Relic Castle, Route 8, Tubeline Bridge, Route 9, Opelucid City (Legend Badge), Legendaries: Tornadus/Thundurus, Route 10, Victory Road, Pokemon League