Secrets

For a minimum of spoilers, you can look at the table of Missions and search any level marked with an *. Those levels each have one secret.

Ever-increasing spoilers after this point...

Secret Places:

Tutorial

Practice Level

Mission 0

Credits

The Core

The Mini Game

The Upgrade Menu

Secret List: (roughly, but not entirely, in the order you should get them)

Dimensional Flux Shields

Secret Counter

9-inch Waste Disposal Cannon

Grenade Reformer

Not-A-Shoryuken

Monkey Climbing Unit

Brain Bomb Decrypter

Expansion Grenade

Gravitic Blue Force Generator Charger 5k

Shield Turret

Advanced Portal Generator

Infragreen Goggles

Hyper Dashing

Pre-bounced Bouncing Boomstick

Mark II Shields

Bullet Jumper++

Jet Pack

Infinite Fuel

Phasing Pack

Shield Regeneration with Emergency Backup

OSHA-approved Bluetonium Recharge

Post-Mortem Blue Force Generator

Atmospheric Absorber

Weapon Systems Mastery

Pyrokinesis

Infuser Pack