You now have every upgrade in the game, but in order to get to the final area you need to collect all 12 of the Chozo Artifact. They aren't actually useful until this point in the game, so you can get them as they become available or wait and get them all at once according to your preference.
Artifact of Truth
You can collect this artifact after getting the Missile Launcher.
Artifact Temple
Go to the Artifact Temple in the Tallon Overworld. (See the Missile Launcher page for directions.) The artifact is located under the central statue. Once you have this artifact you can get clues to the locations of some of the remaining ones by scanning the other statues. You'll be able to scan all the statues after getting more artifacts.
Artifact of Strength
You can collect this artifact after getting the Space Jump Boots, though you can save yourself a trip in getting a Power Bomb Expansion if you wait until getting the Power Bomb.
Monitor Station
Go to the Monitor Station in Magmoor Caverns. Follow the path up to the second level of the central structure. Proceed as if toward the door to Transport Tunnel A, but just at the end of the catwalk, stop and look up and to the Left. There is another platform within reach of a double jump. Jump up then turn around and look up to find a second catwalk. Jump to that and turn right to the third level of the central structure.
Activate the Spinner to lower the collapsible suspension bridge, jump to the ledge beyong the bridge and follow it to the door.
Warrior Shrine
The artifact is on the statue.
Artifact of Wild
You can get this artifact after getting the Spider Ball, but the battle will be easier if you wait until getting the X-Ray Visor.
Sun Tower
Get to Sun Tower via Transport to Magmoor Caverns North. The idea is to get back to the Sunchamber, but you can't get there the same way you did when you got the Varia Suit, so you must follow the route you took on the way out.
Clear the Wasps nests so you explore in peace, then look up at the Spider Ball track to see another gate with four runic symbols. Finding the symbols this time isn't that hard though. First locate the two Cordite wall hangings on either side of the room. Fire Super Missiles to clear them and find two of the symbols. The other symbols are near the tops of two of the four columns around the middle of the room. One symbol is near the Chozo inscription and the other is to the left of the Spider Ball track.
With the gate open you can climb the Spider Ball track, but there are more obstacles on your way to the top. There are gaps in the track and it goes around stone blocks. Drop bombs to jump up the gaps; you destroy a stone block each time in the process but it doesn't matter if you reach the next section of track. Avoid the Oculi circling the blocks because they can knock you down if you bump into one. Continue up the track to the top.
Sun Tower Access
When you enter you may catch a glimpse of a Chozo Ghost. There's nothing else interesting here though.
Sunchamber
Chozo Ghosts attack when you step into the room. When you defeat them, the artifact appears in the giant flower in the center of the room and the vines that stopped you from going through the door back to the Arboretum disappear. Jump to the flower to get the artifact, then take your pick for the route back out.
Artifact of Lifegiver
You can collect this artifact after getting the Gravity Suit. It's in the same place as the Wavebuster so you can combine trips if you want to save time.
Tower of Light
Get to the Tower of Light as described on the Wavebuster page. On the way, you may find Chozo Ghosts in the Ruined Shrine by now. Drop into the water and go through the passage at the far side of the room. Jump to the ledge at the end, then go through the door on the ceiling.
Tower Chamber
The artifact is waiting for you here in a small room.