there really is no good way to decline the king's gambit.
gambits where accepting is better

Sure there is. I can think of 2 perfectly good moves for black right away, and I always play 2.exf4. Unless you mean that the king's gambit is so good accepted that other moves are bad simply because they arent as good. Anyway, I would say 2...d5 is fine and probably extremely annoying for white, and black can even gambit a pawn himself if he likes. 2.Bc5 is also fine and I think tends to lead to vienna game type positions which are theoretically fine for black, which is why the top players dont play the vienna game.

ALWAYS take the first pawn. NEVER take the second pawn.
The same rule applies to rooks.
In the Smith-Morra you should accept the first pawn and if he offers you a second definitely accept that too. Same with the Danish gambit. If you decline out of fear you need to practice some more! Double-pawn early opening gambits are almost never sound.
Actually even the computer will tell you that accepting the second pawn is a mistake.
I doubt it. Run two computers against each other @ equal strength & either black will win or it will be a draw.

Sure there is. I can think of 2 perfectly good moves for black right away, and I always play 2.exf4. Unless you mean that the king's gambit is so good accepted that other moves are bad simply because they arent as good. Anyway, I would say 2...d5 is fine and probably extremely annoying for white, and black can even gambit a pawn himself if he likes. 2.Bc5 is also fine and I think tends to lead to vienna game type positions which are theoretically fine for black, which is why the top players dont play the vienna game.
I think 2...Bc5 is theoretically OK, but it has a reputation of being a bit passive. I don't know any of its theory, so take that with a grain of salt.
If 2...d5, then it depends on how you intend to follow up. 1.e4 e5 2.f4 d5 3.exd5 exf4 4.Nf3 is ok for Black, but then you've transposed to the accepted, modern defence. 1.e4 e5 2.f4 d5 3.exd5 e4 is the Falkbeer counter gambit, which is dubious (probably better for White).

ALWAYS take the first pawn. NEVER take the second pawn.
The same rule applies to rooks.
In the Smith-Morra you should accept the first pawn and if he offers you a second definitely accept that too. Same with the Danish gambit. If you decline out of fear you need to practice some more! Double-pawn early opening gambits are almost never sound.
In the danish, ...dxc3 is good but ...d5 seems to equalize right away with no dangers
Many gambits involve playing a strategically strong move that happens to lose material. It is almost always best to accept such gambits.
Consider the Staunton Gambit 1. d4 f5 2. e4!?. Playing a quick e4 is the most obvious way to punish the Dutch by opening the center and exploiting the resulting weaknesses and advantage in time. There are many variations of the Dutch where White prepares this move in various ways. In each of these variations White incurs some drawback in exchange for playing e4, which is why the Dutch is a playable opening. Black virtually has to take this pawn because otherwise he has allowed White to play his main idea for free.
The Center Game and Danish Gambit illustrate both sides of this concept. White's key idea after 1. e4 e5 is to play d4 to gain control of the center and an advantage in space. Almost every one of the Open Games revolves around the move d4. By playing 2. d4 White cuts through all the theory and just plays his favorite move. If Black doesn't take this pawn he has made a definite concession. Of course the Center Game is not technically a gambit because White is sacrificing not material but rather is offering to let his queen be kicked around. Nevertheless the principle is the same.
But suppose White continues 1. e4 e5 2. d4 exd4 3. c3. This is a true gambit but c3 is absolutely not a strategically good move. It doesn't further development or give an advantage in the center. In fact it blocks the important move Nc3. These are strong signs that declining the gambit is likely to be a strong move. In fact the Danish Gambit Declined is so strong that it has virtually killed the opening. Black just ignores the provocation and exploits White's waste of time by playing one of his own favorite moves with 3. ... d5.
It really doesn't matter whether accepting the Danish Gambit is good or not. That verdict will probably never be overturned because c3 is not a principled move. It is only really helpful if it induces Black to capture it.
Sure there is. I can think of 2 perfectly good moves for black right away, and I always play 2.exf4. Unless you mean that the king's gambit is so good accepted that other moves are bad simply because they arent as good. Anyway, I would say 2...d5 is fine and probably extremely annoying for white, and black can even gambit a pawn himself if he likes. 2.Bc5 is also fine and I think tends to lead to vienna game type positions which are theoretically fine for black, which is why the top players dont play the vienna game.
If 2. ... Bc5 was the main response to the King's Gambit then the Ruy Lopez would be a lot less popular. Playing f4 is an important achievement for White and Black needs tit-for-tat. He either needs to win the pawn or else play his own big move ... d5. Passively declining the gambit is playable but it isn't strong.
The Englund Gambit is unsound. It is best to accept the Evan's gambit. In the Budapest Defense (or gambit) one variation (for Black) is perfectly safe (the mainline) and the other (the Fajarowitcz) is unsound with best play by White. There's the rub - best play. Capablanca played his "siesta variation" of the Ruy Lopez and oppnent's had to prove this pawn sacrifice was unsound. Positionally it strikes back at White's center pawns and thus there is a hint of a positional sacrifice. So we have a wide latitude to decide acceptance or non-acceptance of a gambit.