Firstly, unless it's for Sonic Chronincles, I don't think this is a question for this game.
Secondly, I don't think you have to download them, just read. If you mean AR/GS, then this information may be incorrect. I'm not a cheating kind of person.
As far as I know, two things:
One: A feeling of accomplishment.
Two: You can play as Metal Sonic in the Directors Cut.
In the directors cut, you can play as metal sonic if you collect every single emblem.
First of, let me apologise in advance if this answer is inaccurate, I have not played this game I a while.
You must follow the path, and when you see your blond rival you continue through the metal thing. I highly recommend saving first. Then you will battle entei.
If this is not the answer you were looking for, please say.
Probably whatever you can do right before the last stage of the last planet. You can redo all the levels, battle the boss's, challenge all the stages and so on. I have not yet finished the game(I'm stuck on that darn first level of the final planet) so this information may be incorect.
Whenever you go to a new planet, you will find a wisp. The said wisp will zoom around the screen, and you must touch it on the touch screen to unlock it.
First: I'm pretty sure all Snorlax are big.
Second: You must go to lavander town, after obtaining the silph scope, and go to the top floor. There, you will battle a marowak, after you defeat it you will go to the very top floor. Team rocket will be there waiting for you, and when you defeat them keep on going. You will find a man, then, after the talking, go back down the tower. When you're out, go to a house(Sorry, I can't remember which one, I think it's on the left) and you will see the man. Talk to him and he will give you the Pokeflute. Then, go to the Snorlax and press A while facing it. If that doesn't work, try selecting the pokeflute from your bag. The Snorlax will then wake up and battle you. I suggest trying to capture it. Good luck!
Do you use action replay or any other game ceating devices? They mess up your game like that, so AR or something like that could do that sorta thing. Although it could just be a glitch.
You must go to celadon city and go to the shopping building. Go to the top and press A while facing the vending machine. Then buy a drink, when you next go to the thirsty you will give him a drink and he will allow you to pass.
You cannot catch the ghosts, but you can catch ghost TYPE pokemon. Unfortunatly, unless you cloned it, the master ball is now gone. To catch Pokemon in the tower, you must first obtain the siph scope. You get the silph scope by defeating Giovvani in, I think, the large building that has a sign that says something about silph. Once you have obtained the silph scope the ghosts will be shown as ghost type Pokemon.