The sandy town of Stow-on-Side may have a peculiar name, but there’s actually quite a lot to do in town, and it has a lot of history, including some very old murals in the northwestern part of town that you’ll need to investigate.
As you arrive in Stow-on-Side, you’ll have a look at the small, sandy city and will see where the Stow-on-Side Stadium is located.
Entrance to Town and Daily Shops
There’s a Pokémon Center right near the entrance, which you will likely want to use to heal your Pokémon after your trip through Route 6. Inside, talk to the man at the small table on the left and he’ll give you 2 Fossilized Birds if you’re playing Pokémon Sword or 2 Fossilized Drakes if you’re playing Pokémon Shield. If you picked up the other fossils that were hidden near Cara Liss on Route 6, you can actually make a complete Pokémon now with them—beyond that, you’ll need to visit the Digging Duo in the Wild Area to pick up more fossils.
West of the Pokémon Center you’ll find three stalls, with a Maractus sitting behind one of them. The man in the middle stall will sell you various items, some of them used for evolving Pokémon such as the Reaper Cloth or Magmarizer, while others boost the powers of attacks such as the Metronome or Silk Scarf. His selection changes each day and you’ll also be able to ask him what tomorrow’s deal is to see what’s around the corner. You’ll want to keep an eye out for either the Chipped Pot or Cracked Pot, as these two items can be used to evolve the Pokémon Sinistea into Polteageist, and which item you need depends on whether your Sinistea is authentic or phony, and you won’t be able to tell until after you’ve caught it.
The man to the left of him works in a similar way, except instead of selling you items, he’ll offer to buy an item at an inflated price. The item changes each day and he’ll only buy one of it, but it’s a great way to get some extra 
for some items that you would normally sell in the shop. For instance, if he asks for it, he will be a Big Mushroom for 10,000, which is substantially more than the 2,500 you’d normally get for it. Unfortunately, he’ll only buy one item and the selection changes each day, so it’s a good idea to hold onto at least one of every sellable item (in the “Treasures” pocket of your bag) rather than selling them all, just to make sure you can get some extra money for them.
Before approaching the Stadium and having to fight Hop, you might want to explore the rest of town. It’s your choice, though. Skip ahead a bit if you’d like to get right to that.
Exploring the Rest of Stow-on-Side
In the center of town, you’ll find some kids playing with a Bronzong. To the northwest of that, you’ll find two sacks against the corner of some buildings, and there’s 2 X Sp. Atks hidden over there.
Go directly south of those X Sp. Atks and you’ll find a ladder leading down to a small cliffside that has a few more ladders down there. There are three sacks at the bottom that have 3 Rare Bones hidden between them, and a Rocky Helmet near the ledge after climbing up one of the ladders there. The Rocky Helmet is a particularly nice item for some of your bulkier Pokémon to hold, as any time they take damage from moves that make contact (most Physical-based moves, but not all), the attacker will receive 1/6 of its own maximum HP in damage, which really adds up pretty quickly. It’s a great held item to put on Pokémon that don’t have anything better to equip, too, even ones that aren’t as defensive, but it particularly shines on ones that tend to be slower or more defensive.
In the southwestern corner of town, just to the left of the ladder, you’ll find a path leading behind some buildings that takes you to a back alley area. There are three Diglett back there—a Dug-trio?—along with a path leading to yet another ladder, this one taking you up to the top of the buildings.
Climb up that ladder and keep following the path up there until you find a man sitting on the roof. He’ll offer to trade you his Impidimp for your Maractus. You can catch Maractus on Route 6, although you’ll be able to find Impidimp pretty soon as well, in the Glimwood Tangle—which is actually accessible enough even before earning your fourth Gym Badge to catch an Impidimp inside, despite not being able to explore the full tangle.
There’s also a hidden Metal Coat by the two sacks on the same building as the guy with the Impidimp, and there’s TM74 Venoshock down the ledge to the left. The Metal Coat boosts the power of the holder’s Steel-type moves by 20% and is also used to evolve Onix into Steelix and Scyther into Scizor (Scyther is only available in the Isle of Armor from the Expansion Pass), when you have either hold the item and then trade it. Venoshock, on the other hand, is a Special-based Poison-type move with 65 power that doubles its damage if the target is already poisoned. It’s not bad if you have a reliable way of poisoning the foe, such as Toxic Spikes or Poison Point, but it’s even reasonable enough if you don’t have access to Sludge Bomb or Sludge Wave even without the target being poisoned.
Challenging Hop and Reaching the Stadium
You’ll find Hop waiting for you by the stairs leading to the Stow-on-Side Stadium. He’ll still be acting a bit down from his loss with Bede earlier on and isn’t sure what to do, but he’ll challenge you to a battle if you accept (you can decline to go heal if you need).
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Hop has decided to mix his team up by quite a bit, so if you were expecting him to lead with that same darn Wooloo he’s been known for leading with, you’ll be in for a surprise when he sends out his Cramorant. None of his Pokémon should be that difficult, but they do offer coverage for different types.
His final Pokémon (though not necessarily the last one he sends out in battle) will be the middle evolution of whichever Pokémon he started with, which varies based on which Pokémon you picked at the beginning.
After you’ve beaten him, he’ll be a bit frustrated, but will then regain some confidence and head off to catch some more Pokémon. You’ll then be approached by Opal, who was apparently watching the fight the whole time. She’ll give you either Bea’s League Card (Pokémon Sword) or Allister’s League Card (Pokémon Shield), depending on which version you’re playing.
Climb up the stairs, but be on the lookout for another ladder on your right. Climb that ladder and you’ll find a Cracked Pot on the roof up there, which is one of the items needed to evolve Sinistea (it evolves Phony Sinistea, which is the most common type).
Anyway, the Stow-on-Side Stadium is right ahead. The Gym Leader and the type used in the Gym Challenge vary based on whether you’re playing Pokémon Sword or Pokémon Shield. In Pokémon Sword, you’ll face off against a Fighting-type Gym and the Gym Leader Bea, while in Pokémon Shield you’ll find yourself fighting a bunch of Ghost-type Pokémon and the Gym Leader Allister instead.
As such, the Stow-on-Side Stadium is split up into two separate pages, so choose the one that matches the version you’re playing.
Additionally, even though you’ll need to head there after getting your Gym Badge, if you want to visit it early, you can climb the steps to the west and find a hidden Max Revive in the first circular landing area on the west side, against the wall facing the sea. There’s a mural at the top of the steps, but it’s just there to examine and offers no items or anything else.