Metroid is brimming with secrets and tips to discover. Check out some interesting points about the game you might not have known about.
In the area where you find your first Ice Beam, climbing back up to the ledge where you can't make it with a normal jump can be done without using the reappearing block to propel you up. If you use the bomb jump trick (drop a bomb, stand up and then jump while in mid-air from the bomb blast), you'll easily clear the ledge without having to use the reappearing block.
If you're playing for a fast time it's a good idea to go for Ridley first before Kraid and don't bother finding the Wave Beam, the 50 missiles you'll get from Ridley will help against Kraid. Also another note, if you're playing the Famicom Disk System version of the game, Ridley's fireballs will alternate between short and long bouncing patterns, but in the US version (most likely PAL too) they'll randomly change between short and long bounces.
Here's a few tips for taking on Ridley:
One of the fastest ways to beat Kraid is to freeze his three spike missiles after they launch away from his body, then fill him with missiles before they unfreeze. Keep this up while dodging the flying bombs to take him down pretty quickly. Fake Kraid can also be beaten with one Screw Attack.
A good trick when fighting Mother Brain is to freeze the bubbles that fly around (only three ever appear at one time), this will give you around 7 seconds to rip into Mother Brain.
Strangely if you make it out of the final elevator too fast the game will think you cheated and gives you the worst ending for the game. If you make it to the top of the shaft too fast wait around 100 timer units before hopping onto the elevator.
For a faster completion time it's better to go after Ridley first and then take care of Kraid second. If you manage to jump right over Ridley and stay behind him you'll be able to tear away with the Wave Beam without the threat of taking heavy damage from his fireballs.