A lot of baking soda. A few cups of colored vinegar.
There was no instruction whatsoever, just me saying it was baking soda and vinegar.
So how were we learning about our five senses? We talked about what we saw, what we heard, and what we smelled. As you can see below, there was a lot of touching as they got their hands right in there. They did that on their own, I did not suggest it.
And at some point...one child licked something off their hand, so taste was involved. And he was very descriptive about that taste!