You can make a permanent rainbow with water, clear nail polish, and black construction paper:


Then there's the ever-popular color-changing milk experiment:

Window hangings made with yarn and squares of paint chips:

And my absolute favorite, the rainbow cake. I found the idea here, but it seems my round cake pans are on the missing list, so I improvised. It won't be the same effect when cut into, but pretty all the same!
I took some shortcuts. I grabbed one box of white cake and followed the usual instructions (eggs, oil, water).

Separate into six bowls and color with gel food coloring.

I put mine in a regular 9x13 pan.

And baked, using the instructions on the box.

It's still cooling now, but we'll be frosting it tonight. We've got rainbow sprinkles to add the finishing touch!