Use this guide to help you find all the hidden items. At the mission select screen you can see your progress and what you need to find in each level. Here are some general tips for finding them if you don't want to follow a walkthrough:
- If you see pressure switches, go there first as there will be a hidden area.
- Keystone rooms must be entered and all enemies defeated to count, you can't just open the door.
- Some hidden items will be inside breakable objects, so smash everything you can.
- Some pressure switches might be on walls, so punch them to activate them.