Hi,
Not sure if these are any good, but...
This one's pretty familiar, at least for artists. What do you see?

Negative area. See what's not there.
This one's a little bit complicated. Position your head to about 30cm distance of the monitor. Close your right eye, then look at the spot on the right. Can do the same with left eye closed, in that case look at the spot on the left.

Blind spot. There are no light receptor cells where the nerve connects in the eye.
Finally a banana. Stare from close range at the small yellow spot in the middle for a minute or so. Don't let your eyes wander. Then close your eyes and cover them with your hands.

Overburdening light receptor cells causes a "backfire" showing the inverse image.