S.S. Aqua
Go down to the S.S. Aqua ship in Olivine city after you beat the Elite Four once. There you can get the National Pokedex.
Pokewalker
Use your Pokewalker. The Pokewalker is a device included in "SoulSilver" that allows you to record how much you walk in the actual world. It then awards you "Watts" based on how much walking you did. You can then use those Watts to trade in for areas in the "SoulSilver" game which would be unavailable otherwise. In those areas you can catch Pokemon that would be unavailable otherwise. For instance, for 200 Watts you can get to the Beautiful Beach; for 2,000 Watts you can get to the Blue Lake Area.
Game Boy Advance Sounds
Go to the Celadon Condominiums next to the Pokemon Center in Celadon City. Then use the elevator to get to the third floor. After you leave, the old man will tell you about GB sounds. After collecting all the badges, talk to the man next to the globe and he will give you the Game Boy sounds advice. This will allow you to play the Game Boy sounds on "Pokemon SoulSilver."
Hidden Unknowns
Collect all the Unknown Pokemon in the ancient temple. To do this, you must complete the alphabet, for there is one Unknown Pokemon for each letter. After you do that, the Unknowns "!" and "?" will appear in the hall temple -- collect them too.