After beating Misty, the whole Kanto opens up, with the only exception being Surf areas. So there was a lot Pokémon to catch.
But first, I went back to Pewter City and obtained Old Amber and Flash. Then I went through Routes 9 and 10, caught Voltorb and got pass Rock Tunnel, where I caught Machop and Onix. I also got Growlithe, before arriving in Celadon City. This city became my main hub for going around Kanto. It was the place, where I could buy elemental stones, which means I could evolve Pikachu, Gloom, Weepinbell and Growlithe. I also picked up Eevee and since I can breed more later, I decided not to get more of them from my other saves. Another reason why Celadon was my hub was a group of bikers just west of Celadon, that only had Poison and Fighting-types, so they were easy to beat with Pokémon like Alakazam.
I did one round around the region, beat every possible trainer, including Team Rocket, caught way too many Pokémon to list here, got Good and Super Rod, Exp. Share and Amulet Coin, caught even more Pokémon and then got to grinding.
Good thing about this part was the already mentioned bikers, since they were easy to beat, but still gave out a lot of EXP. The worse thing about this part were some encounter rates. Grimer in Celadon City had only 1% rate, there were Pokémon in Safari Zone with 4% rate and I still had to get Porygon and evolve Dratini. Which wasn't that hard in the end. I had enough money to just buy enough coins for Porygon. And Dratini leveled up fast thanks to the bikers. The version exclusives were as annoying as ever, but hopefully, there won't be many of them in later parts.
In the end, I almost completed the PokéDex and don't even have 3 badges. Which means the next few parts will be shorter. So, next Gym Leader will be Koga, since beating him will unlock Surf outside the battle, meaning I'll be able to get more Pokémon, including legendary Pokémon Zapdos.