After you travel through Diglett's Cave, your partner pokémon learns Light Up, and you go to Route 9 on your way to Rock Tunnel.
Explore the Route
In the grass in this area, you can encounter Spearow, Raticate, Nidoran♀, Nidoran♂, Nidorino, Nidorina, Rattata, Fearow, and others.
East of the west entrance to Route 9, you can battle Picnicker Alicia, who has a level 21 Eevee and a level 21 Gloom. You get 420 Pokédollars and 2 Great Balls for winning.
Go south and jump down the ledge. Go south again to get a Super Potion.
Go east from there to battle Hiker Jeremy, who has a level 22 Onix. You get 880 Pokédollars and 3 Poké Balls for winning.
Go east, then you can go north, then west, to reach the grassy area. There is an Ether in this area.
West of there, you can battle Bug Catcher Conner, who has a level 19 Paras. You get 228 Pokédollars and 3 Poké Balls for winning.
Go back to the east, then go southeast. There are two paths going to the east. The south path is a dead end, but there is a trainer battle and hidden item there. If do you use the south path, you can battle Hiker Alan, who has three level 22 Geodudes. You get 880 Pokédollars and 3 Poké Balls for winning.
To the east of Alan, there is a hidden set of 5 Pinap Berries.
After battling Alan and getting the hidden berries, go west until you can go north, then use the upper path to go east this time.
The path will lead you to Camper Drew, who has a level 21 Sandshrew and a level 21 Raticate. You get 420 Pokédollars and 2 Great Balls for winning.
Go west from Drew and you can battle Youngster Chris, who has a level 20 Diglett and a level 20 Fearow. You get 400 Pokédollars and 3 Poké Balls for winning.
To the left of Chris, there are five Poké Balls.
Go back to the east, then go north from the Camper to find a Super Potion.
Go east from there and you can battle Black Belt Brice. He has a level 23 Mankey. You get 644 Pokédollars and 3 Poké Balls for winning.
Go east, and jump down the ledge. To the left, you can battle Picnicker Caitlin, who has a level 21 Goldeen and a level 21 Pidgeotto. You get 420 Pokédollars and 2 Great Balls for winning.
Keep going east from there to reach Route 10.