The NES version of Hook is a pretty terrible game, but if you absolutely must see the ending, I have some good news, I guess.
First, go to the title screen.

Then grab Controller 1 and press Right, Up, Up, Down, Down, Right, Left, Up
Done right, you'll hear a sound, then hit Select to see the ending... which really just consists of two screens of text with a Tinkerbell flying around in front of them.


Or, you could just start the game. If you do, you can use the compass to point to a stage and hit the B button to move around on the map, and the A button to play any stage you like.

But since you've already seen the ending, I'm not sure why you'd want to do that.
Kid Niki is full of hidden passages and bonus stages, some of which are... different.
Like take this spot in Stage 4, for example. It's the platform directly after the second moving platform.

Jump up a few times and the next platform will turn into a fish!

Walk off the platform, directly into the fish's mouth and...

You're in!

You traverse the length of the fish (there's nothing in it but bones), and you come to the exit

Well, how else did you expect to get out of a fish?
In Tetris, you occasionally will have to maneuver pieces so that they fit in spaces where it doesn't initially look like they're going to fit.
Like this T-piece, for example.

It would totally fit in that hole on the left column if some of those pieces weren't in the way.

But if you let it get partway in

And then rotate it at the last second, it slides into place
Which is easier to show in animated .gif form

This is a pretty useful maneuver, since it lets you slip those T-pieces in places where they wouldn't normally go.
Marble Madness is tough, probably to make up for it also being kinda short, so you're going to be running out of time frequently, until you've mastered the game... which might take some time.
In the meantime you could start up a two-player game (even though you're only going to be playing single player)

Then race normally, letting Player 2 just sit there for a little bit

As you race to the finish line, the second player will be scrolled off the screen, and brought back closer to where you are, incurring a 5 second penalty each time. But if you can get it across the finish line before its time runs out (which should be totally doable on the first race), whichever marble crossed the line first will get a five-second bonus at the beginning of the next race

Which doesn't sound like much, but trust me, you're going to need all the help you can get.
You remember Karnov, right? The muscular fire-breathing circus strongman? He has his very own NES game, and it's actually pretty tough. Tough enough that if you make any kind of progress that you might not want to turn off your NES when you're done because you'll have to start over from Stage 1 if you do.
Or do you?
If you go to the title screen

And on Controller 1 hold Right + Select + A + B, and while holding all those down, press A on Controller 2 one time for each stage you want to skip, and while still holding down all those buttons on Controller 1, hit Start on Controller 1 (you might want to enlist the help of another hand or a foot).
Done right, you'll be whisked away to the stage of your choice

The only problem is that you won't have the benefit of having collected the items scattered around the levels that you skipped, but that's a small price to pay, right?

You might remember some time ago that I covered a method for continuing in Super Mario Bros. But would you believe that this works or other games, too? Because it totally does.
Like Kung-Fu Heroes.
Get the Game Over screen (and you will, trust me)

Head back to the title screen

Hold A and press Start, and wham!

Back to where you were to try again!
Which is much faster than trying to work your way through the beginning stages again.
Play James Bond Jr. long enough and you'll eventually see this screen.

It would be pretty awesome if there were some way to put off seeing that screen for as long as possible, right?
And there totally is!
If you go to the passcode screen and enter this code:

The timer will go up to 99:99 and your bullets will be maxed out!

Which should go a long way toward delaying that screen from appearing.
Knight Rider is an OK game, but not one that I was ever very good at. So, I got to see this thing a lot.

After failing to do whatever it is I'm supposed to do in the game a few times, I just hold down the Select button on controller 1 and hit the Reset button on my NES
Then, the ending starts to play

Just be aware that it's one of the lamest, most unsatisfying endings you'll ever see. Which looks to be a pretty big letdown if you earned it the hard way.
Let's say that you're playing Bionic Commando. You go through Area 13 and get the intel that you need to take Flares into Area 4 so you can see.

Then you get to Area 4 and then either forget or are uncoordinated enough that you go in without them. What do you do?
Well, you could try to play the stage in the dark, but trust me, that's virtually impossible to do successfully. Or you jump into a spike-filled pit to sacrifice all your lives, then continue (if you have any) and try again. But there's a more elegant way.
Hold down the A + B buttons

And press the Start button.
Done right, you'll be taken back to the map screen!

Ready to try the stage again!
The first few boxers in the Punch-Out!! games have techniques that teach you how to play the game better.
Like Don Flamenco, for example.

The first time you fight him, you have to throw a punch (which he'll block), so he can counter it (he's kind of a showoff)

Dodge that and he'll be vulnerable to taking punches (like everyone else in the game).

But, unlike everyone else in the game, if you keep alternating left and right punches Don will never recover, letting you pummel him until he runs out of stamina and crashes to the mat.
Even better is that each time he gets up he gets a little more aggressive and throws the first punch, which you dodge, and then you can immediately start with the alternating jabs again.
Which means that you're going to win in record time.

Just be aware that this doesn't work the second time you have to face him.