Successfully complete the game. Then, enter to a warp room to access Boss Rush mode.
Crazy mode
Successfully complete the game. Then, start a new game and enter "@CRAZY" as a name to play on the Crazy difficulty setting.
Moebius Brooch
Successfully complete the game on the Crazy difficulty setting to unlock the Moebius Brooch at the shop. When this item is equipped it allows you to do special moves without using hearts.
Musical Chairs
As you may of noticed, throughout the game there are various chairs you must sit in. If you happen to sit in them all, the room inside Cordova Town (the one that is pretty colorful with blue cloudy skies twirly around while little wierd rabbit/bug creatures are flying around with a target sign on their gut) will obtain a special chair in the center, though it really doesn't do anything for you, it's a nice little extra quest that you can do.
Optional Boss Pain
In order to reach the optional bosses -- Legion and Nuculais -- you first must obtain the fifth Innocent Devil (Devil Type). With this, head back to Garibaldi Temple and slide under the crack in the wall back near the area with the four courtyards and Fencers. Proceed through the hallways, avoiding the Blood Bones and eventually you'll be in a hallway that is living. Save before you enter the boss area and make sure you're at least at level 50 or expect a tough fight.
Play as Trevor Belmont
Successfully complete the game. Then, start a new game and enter "@TREVOR" as a name to play as Trevor Belmont. Trevor cannot craft items or use innocent devils.
Sky high
Successfully comlete the with Trevor then start a new game with Hector. Obtain the dark crystal (rare material) then beat Dullahan Lv:49 (9th level boss) and he will drop something when you obtain it, it will say sky high which means you can fly. To fly press and hold the gaurd and the jump buttons.
Sound mode
Successfully complete Boss Rush mode to unlock the music box in the warp room. This item allows access to the "Sound Mode" option to play various music from the game.
Tower of eternity
The tower of eternity is located in the forest of jigramunt(near the warp room area). Once you get in it theres no turning back. You have to pass 50 floors each with a different kind of enemy(you get a potion after passing 5 floors). After you passed all the floors and reach the terrace, you'll find an item called ''kit bag''. Now jump out from the terrace and you will come back to the ground unharmed.
XP Monopoly
In the Forest of Jigramunt at the starting point head East. Once you have cleared the Vassago Lv.25 and two Sniper Orc Lv.24, the Spirit Lv.19 enemy will continually respawn in pairs. They drop a ton of Evo Crystals if you kill them with one blow. Use the Brooch item to give all the experience to your I.D. This is an easy way to see all of the forms of the I.D. and to level up the ones you'll use the most.