Honestly, there isnt much to do. You can visit the Battle Tower on one of the Sevii Islands, you can complete the Pokédex, you can just train a bunch, or rechallenge the Pokémon League. If you have a DS and a DS Pokémon game (fourth generation, this doesn't work with Black/White) you can migrate your Leafgreen Pokémon to that game. Like I said, there isn't much to do. CareyPrice31FTW
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When you leave Pewter City after defeating Brock.
You can defeat the Pokemon League by defeating the Elite Four and the Champion in Pokemon Soul Silver.
All you have to do is just keep going back and defeating the elite four!! $FR3CK$
After defeating Blaine at Cinnibar Gym, you must return to Viridian City. The gym will now be open, and after defeating the leader, you'll get the Earthbadge.
You speed through the credits and re-open your game. Then you can continue to catch and train pokemon.