Walkthrough
If you decide not to take up on your Rival's offer to take you directly to Rock Tunnel after learning the Light Up Technique on Route 2, you will need to take the long way there. However, this will actually be all the more beneficial to your Pokemon's growth. Just east of the entrance to Diglett Cave near Vemilion City is Route 11. This path is full of both Trainers and Wild Pokemon, and it will lead you to into Route 12. Unfortunately, Route 12 will be blocked by a sleeping Pokemon. Because of this, you will need to then head all the way back to Cerulean City (which isn't too far). Once there, you will now be able to head east on Route 9 since you now know the Chop Down Technique and can cut the trees that once blocked the path.
Wild Pokemon
Grasses:
Pidgey (25%): Levels 13-18
Rattata (25%): Levels 13-18
Drowzee (25%): Levels 13-18
Pidgeotto (10%): Levels 13-18
Raticate (10%): Levels 13-18
Mr. Mime (5%): Levels 13-18
Grasses - Special Spawns:
Chansey: Levels 13-18
Waters:
Tentacool (30%): Levels 13-18
Horsea (30%): Levels 13-18
Magikarp (30%): Levels 13-18
Tentacruel (5%): Levels 13-18
Seadra (5%): Levels 13-18
Sky:
Pidgey (60%): Levels 3-56
Pidgeotto (35%): Levels 3-56
Pidgeot (5%): Levels 3-56
Sky - Special Spawns:
Dragonite: Levels 3-56
Charizard: Levels 3-56
Articuno: Levels 3-56
Zapdos: Levels 3-56
Moltres: Levels 3-56
Trainers
Coach Trainer Will: Level 21 Rattata
Sailor Dirk: Level 18 Poliwag, Level 18 Machop
Youngster Eddie: Level 16 Nidorino
Gentleman Wilco: Level 18 Vulpix
Engineer Bernie: Level 18 Magnemite, Level 18 Voltorb
Gamer Hugo: Level 26 Diglett
Engineer Braxton: Level 18 Voltorb
Sailor Yasu: Level 18 Slowpoke
Lass Tomoe: Level 16 Jigglypuff, Level 16 Nidorina
Obtainable Items
Eevee Candies: Defeat Coach Trainer Will
Super Potion: In Middle Of Route
Poke Balls: Defeat Youngster Eddie
Poke Balls: Defeat Gentleman Wilco
Lure: Near Gentleman Wilco
Super Repel: Far Northeast
Poke Balls: Defeat Engineer Bernie
Poke Balls: Defeat Gamer Hugo
Poke Balls: Defeat Engineer Braxton
Poke Balls: Defeat Sailor Yasu
Poke Balls: Defeat Sailor Dirk
Poke Balls: Defeat Lass Tomoe
Great Balls: Near East Gate
X Defense: Near Game Hugo
Revive: Hidden Within Northeast Rock
Post Game
Walkthrough
After defeating the Elite Four and starting the Master Trainers Challenge, you will find a number of Master Trainers on Route 11. In total, there will be three Master Trainers to defeat.
Trainers
Bug Catcher Bryson (Master Trainer): Level 65 Metapod
Hiker Paul (Master Trainer): Level 70 Sandshrew
Blackbelt Kano (Master Trainer): Level 75 Primeape