One favorite activity is painting. My daughter loves liquid watercolor paint, and we often have drawn outlines and then later filled them in with color. One morning, I encouraged her to try the opposite. Inspired by a field of wildflowers we'd seen, she started with the color:
Then, once the paint was dry, she added black Sharpie marker as an outline. It was fun to see the outcome!
Later, inspired by an idea we learned from The Artful Parent, we experimented using glue, salt, and liquid watercolor. It was such fun to watch the watercolor travel!
At Mucky Kids Art Studio in Cambridge, we experimented with droppers instead of paint brushes. They had pieces of felt out, which absorbed the paint differently than paper. It's amazing how small tweaks to usual activities can lead to a very different outcomes!