I like this color mixing activity at Delightful because it's simple to put together and puts the child in control of the experiment. It's a nice quiet activity too - perfect for older kids to do while younger siblings are napping.
Origami boxes are simple to make and I love that they are practical too! This colorful set above is from Good + Happy Day and you can get instructions on how to make the boxes at Origami-Instructions. (Use papers of graduated dimensions to create a nested set.)