We all know coloring works on fine motor skills. However, there are ways to develop gross motor skills at the same time. Here's a few examples:
1. We have an art desk that has the chair and inclined desk attached. We got it for her when she was 18 months old and she still has to sit/stand on her knees to color. Believe it or not, this is a gross-motor activity. Find a place your child can reach by sitting on their knees (or standing on their knees) and have a spot to color. That may be knees on the floor and coloring on the seat of a chair, or taping a piece of paper a little higher than normal on a wall or door. This way they have to use their legs, hips, and stabilizing muscles to color.
2. Taping paper on the floor and letting them color. I tape butcher paper (huge sheets of it) to the kitchen floor when I cook sometimes and let her color. They have to get on their hands and knees and color. When they do this, they use arm and leg muscles as well making their hip and shoulders stronger. Keep and eye on them, as my daughter tires out this way and ends up laying belly down and coloring with her eyes about a millimeter away from the paper.
3. Thanks to our school OTs that recommended this! Tape a piece of paper on the wall or door or put a piece of paper on an art easel for them to color on. (You can also do this with a chalkboard.) Place pillows or cushions on the floor where they'll have to stand on them in order to color/draw/paint. The soft floor provides a slightly unstable surface to stand on and allows them to work muscles while trying to stabilize themselves by shifting their hips and weight. Kind of like the idea of walking on pavement vs. walking in the sand.