Main Plaza is a room in the Chozo Ruins region of Metroid Prime.
Without Grapple Beam
It is possible to get the Missile Expansion on the ledge without Grapple Beam using a dash or a jump.
For the dash method, you want to stand ontop of the tree where you would normally use a Super Missile to break open. Next, lock onto the orange highlight down by the Missile door, and do a dash while bending it slightly upright.
For the jump method, simply climb to the top ledge where the Missile door lies. Simply do an R-Jump to get over. An L-Jump works as well.
Without Boost Ball
You can easily get the halfpipe Missile Expansion without Boost Ball. Simply walk up the halfpipe and use Space Jump to collect it.
Early Energy Tank
With Space Jump, it is possible to obtain the Energy Tank in this room without having to go through Vault. Stand on the highest step by the ledge, then perform an L-Jump to get over there. It is important to note that you should re-hold L when entering the alcove.
Alternatively, you can stand on the rock that's in front of the step and hold L for the entirety of the jump. It's a little bit easier, too.
- Get up to the platform where your first missile expansion should have been grabbed. Turn around to face the grapple beam point. Look slightly to your right and you will see a red plant. Carefully jump on it. Turn around roughly 170 degrees and there should be a rather blocky part of a pillar that looks like you can jump on it. You can, so jump on it. Jump straight up and push into the wall that's to your left. You should land on another platform. Take one last jump over the wall into the secret world.
- Alternatively you can do two things. Get up to the same platform the first missile on it. This time, look towards the door where the morph ball was acquired. There should be another ubiquitous "red plant" that you can hop on. You'll need to do a ghetto jump off of the sand pile. If you make it on the plant, turn 180 and jump onto the wooden pole sticking out of the wall. Turn to your right and jump to your freedom. Otherwise, you can jump up to where the early Energy Tank is, do a crazy long L-Jump to the tree twisting out of the ground then do a scan dash out of the level.
- This was secret world #9 in the original numbering scheme.