I feel you! I just came up with a bunch of indoor activities for my kids yesterday! We made our own playdough because my girl claimed she was bored, and I didn't want her boredom eating into my sanity
The ingredients are probably already in your pantry:
You'll need 2 cups of flour, 1 cup salt, 2 tbsp cream of tartar, and 1 cup warm water. Mix the dry ingredients together, then add in the water and stir over medium heat until it forms a ball of dough. That's it! I divided the dough into three portions and added liquid food coloring to it. The kids loved the squishy texture and played with it for more than an hour. Then I let them make sculptures using plastic bottles and cardboard rolls as their 'tools.' It was very engaging, inexpensive, and messily fun!