Maria
In the second stage, get to the portion where the Minotaur chases you. You'll see candles you can whip. Hit all of them, and you should get a key. Pick it up, and it'll become a sub-weapon. When the chase is over, go to the end of the sewer. You'll see a locked door, use the key and you'll unlock Maria.

Alternate Level 3
During stage 2, when the Minotaur chases you, drop into the first hole in the ground, and you'll land in a sewer. Finish the stage through the sewer, and you'll get the alternate version of level 3.

Symphony of the Night
Go to the alternate version of the third level in the 3D Rondo of Blood. Go to the very top level of the stage, where the bone snake enemy is. To the left of the snake you'll see a green cage object hanging from the ceiling. Hit it with an axe, or use Maria if you have her. It'll unlock a platform which you can use to climb further up. Eventually you'll find a white tombstone. Break it and pickup your reward.

Rondo of Blood (Original)
In the fourth stage of the 3D game, when you're near the end, you'll see an area where you'll be able to destroy an entire lower part of a wall. A bomb will appear. Hit the bomb and the rest of the wall will break. Go inside the new room, up the platforms, kill the enemies, and at the end you'll find a pickup that unlocks the 2D original.