Remark: Anything flashing is an item (good for you). Some items produce points.
$ is a lot of points. Every 20.000 points you get an extra life.
Reminder: in order to unlock secret items you need to:
1. Hit the skull with the gun -> unlock simple secret items
2. In a non-boss level, kill the skull and complete the level without dying after the kill (repeat this in more than one level) -> unlock some special secret items
3. In a non-boss level, destroy the missile bases (holes) and complete the level without dying after this (repeat this with more bases for more items) -> unlock even more special secret items
The most fun item is the zombie item (it looks like a flashing yellow enemy): it will trigger more items and it will produce points for you but DEAD enemies will come to life to chase for some time.
The most powerful item is the red S pill, which is like a super freeze (white pill) + gun + invincibility.