Battle every trainer you can encounter and catch every Pokémon that can be found on all of the routes up to that point and then if you aren’t strong enough catch catch catch in the closest patch of grass to the gym you can find and then run over there and kick ass. I recommend this grind because you get to really explore the game and still get a sense of adventure rather than run around the same spot until you’re a beefcake (or you’ve trained beefcakes I guess) stare unending, eyes never blinking.