Super Mario World is chock full of these bonus rooms. Like this one that has rows and rows of ?-Blocks.

If you can hit them in the correct order (which is random), a 1-Up will pop out, but if you hit them in the wrong order

You get nothin'.
That's why it's recommended to enter these rooms with a Cape.
If you can manage to hit the bottom-left corner of the blocks, you'll be correct every time!

Which will allow you to get a few free 1-Ups for your troubles

And timing is key. If you hit the side of the blocks, it might not work (but, since it's pretty much random anyway, it might).
Good luck!
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.
Been playing the Super NES port of Mortal Kombat 3 and been getting bored with it? (And at this point the game's life, who wouldn't be?)
Well, then, let's tweak a few options, shall we?
First, go to the 'Start' Screen

And press Up, Up, Down, Down, Left, Right, A, B, A on Controller 1. Done right, a new menu option will appear, Kool Stuff

With several things to tweak.

Not enough?
Well, then go back to the 'Start' screen and press Select, A, B, Right, Left, Down, Down, Up, Up on Controller 1.
Another option will appear, Kooler Stuff

With lots more options to flip

Still not enough?
Okay, go back to the 'Start' Screen and press X, B, A, Y, Up, Left, Down, Right, Down on Controller 1 to get the final menu choice, Scott's Stuff

Which has yet more options for you to play with

Now things will start getting interesting, so go nuts flipping those options!
Thanks, reader EASY, for suggesting today's tip!
The first NBA Jam game had a few hidden characters, but it has nothing on the Tournament Edition of the game, which went way over the top.
For instance: if you enter your initials as GRM

And your birthday as OCT 31


Your player turns into the Grim Reaper! Who looks kinda funny rather than scary.

Just be aware that this doesn't work on all revisions of the game, just v.1 and v.2.
Playing Super Puzzle Fighter and tired of playing as Dan (or one of the regular characters)?
Well, then I've got a present for you!
Start the game as Player 1

Then, hold down the Start button and press Down, Left, Down, Left, Down, Left, Down, and then press and hold Left on the Joystick (while still holding Start, remember). While holding all these down, press one of the Action Buttons to select Morrigan. Done right, she'll turn into Akuma!

Who cuts a much more imposing profile.

And should make the game a little more fun.
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.