Secrets

Instant Max Drive
If you want you drive bar maxed, all you have to do, is, Go to the Underworld tournament. Talk to Panic, after talking to him, A screen of all Tournaments pop up.
Go to any one, and once you enter the first battle, Your Drive Bar will be completely filled. Pause, and select quit. When you leave, your bar is still full.
This will help you level up forms and summons quicker.
Also, if you return to the world map while in a drive form you will have a full drive gauge.

Quick Summon/Form Levelling
In Space Paranoids, just use any form or summon, and let the form/summon meter count down until it’s almost gone. Then, immediately board the Solar Sailer. Your form/summon will be cancelled, but your drive gauge will be returned to its original state. Ride the Solar Sailer, then use the form/summon again. After the ride is over, head back and all of the enemies will return. Repeat until forms and summons are maxed. Obviously, this will not work for Final Form, since there aren’t any Nobodies in Space Paranoids, but the other forms and the summons can be levelled freely, without any pesky drive-gauge refilling. It’s possible that this trick works in other areas, too, but I have not explored it (the Black Pearl is an idea though).
Also, to level summons specifically, the experience counter seems to increment when the summon meter ticks down a notch. If your timing is good, try to kill enemies with drive orbs right when the meter is going to drop a level. If you can keep the meter almost full as you collect the orbs, you can make it oscillate between two numbers, netting extra experience for your summons. With this, you can get over 30 exp every time you use a summon.

Unlockables

Cup Unlockables
The Paradox Cups can be played by talking to Hades in Hade’s Chamber.

Special Models
These are special models of gummi ships you can get by doing the certain gummi missions and getting S ranked on them.