give him the ruby on island six, then you can catch mew and mewtwo
The reggis are not available in leafgreen you will need Ruby, Sapphire and Emerald to give you them.
You also find the sapphire and give them both to Celio.
Pokemon cannot be transferred from Diamond to Leafgreen. This can be done backwards, however as Ruby, Sapphire, Emerald, Firered, and Leafgreen Pokemon can be permanently transferred to Diamond, Pearl, Platinum, Heartgold, or Soulsilver.
Trade it to ruby,sapphire or emerald have them evolve your evee at night then give it back to leafgreen.
Give them to Bill's friend in the big Pokemon Centre on One Island!
In Pokemon LeafGreen Oak will never give you a Mudkip, Treeco or Torchic. This is primarily due to Leafgreen and FireRed being in the same generation as Ruby and Sapphire which has those starter Pokemon.
Mudkip can only be owned in Pokemon FireRed & LeafGreen versions by trading from Pokemon Ruby/Sapphire/XD/Colosseum. Trading from these games requires the National Pokedex, the Ruby and the Sapphire. You must give the Ruby and the Sapphire to Celio at One Island. Trading requires a Wireless Adaptor.
It allows you to link with Hoenn games :)
After you have obtained and restored the Sapphire gem, you now are given the ability to trade with trainers from the Hoenn region. Mind you these need to be your real life friends, and they can give you all the Pokemon from Ruby, Sapphire, and Emerald.
You can't give your Pokémon a haircut in Sapphire.
probably action replay but you can just buy a firered leafgreen sapphire or ruby and just save the masterball you get from them and give them to one of your Pokemon and migrate it
There's not much after that. You can go to seven island and catch all the unown, but I'm not sure if that does anything.