Have you ever made rainbow cupcakes? They just taste happy! They would also be a perfect rainy day activity to do with kids.
All you do is take your favorite white or yellow cake mix (I do mine from scratch to avoid all those chemicals & preservatives) and divide the batter into bowls. Then you add your food coloring to each bowl to get your desired color.
When you are adding your colors into your cupcake liners you will want to use a fork and let it drizzle into each cup. This is so your colors don't mix and they stay in layers. It's a little time consuming but it's raining outside so you probably have nothing else to do anyways...
Bake like you normally would and top with some fluffy white icing
Make sure you use white liners so you can see all your hard work!