You can go and get a Kanto starter from Prof. Oak.
You can go and get a Hoenn starter from the Hoenn champion in Silph co.
You can rematch the gym leaders but they're in various places.
http://www.serebii.net/heartgoldsoulsilver/gym-rematch.shtml
Here's a guide on where to find them.
You can catch the rest of the legendary Pokemon.
If you catch a Ditto you can breed it with a male Pokemon and still get an egg.
You can get a free Shuckle from a person in Cianwood City, a free Drantini from the old man in the Dragon's Den and a free Tyrogue from the karate master in Mt. Mortar. (These Pokemon can be acquired before beating him, but they're often missed)
You can rearrange the Safari Park.
You can do the Ruins of Alph quest.
You can partake in bug catching contests in National Park.
You can partake in the Pokeathlon Dome.
You can partake in the Battle Frontier.
That's all I can think of to this moment.
go to the tubes in ceruelian gym and find the machine part and take it too the power plant
^^Nerds.
It attracts a ledendary pokemon.
You can't find Togekiss in Pokemon Black or White. You have to transfer from Pokemon Soulsilver, Heart Gold, Pearl or Diamond.
get an action replay and find a cheat for it (i forget what it is called) but there are 2 types, heart scale and master ball. i prefer the heart scale cheat.
You'll have to transfer one from Pokemon Heart gold and Soul silver
They r dug up underground. They are hard to find
I've played Crystal and the official Heart Gold and if my speculations are correct, the shady character(Rocket goon) should be in the Cerulean City gym.
in dragons den
You ARE Dawn in Pokemon Pearl if you are the female character.
you have to use hm rock smash on rocks to find them.
On wild Pokemon and serval locations.
It attracts a ledendary pokemon.
you find the shady guy downtown near the fashion inc.
You can find them in the Underground
underground
you can find information on Pokemon heart gold and soul silver on this website www.bowserrules.piczo.com
To find heart scales on Pokemon diamond/pearl you need the explorer kit to go underground. once you are underground look for cracks inthe wall, and eventully you will find a heart scale in the cracks.
pokemon heart gold