Use to make playdough or to make your own grain-free baking powder. There's a link and recipe below for how to make it, plus some tips on how to use it.
To make 1 tablespoon of baking powder:
Measure 1 teaspoon baking soda and 2 teaspoons cream of tartar into a bowl.
Mix until thoroughly combined and use right away.
If you need more, you can just double the recipe. If you need to store it for some reason, just add a teaspoon of corn starch (so that it doesn't clump up), and store it in an airtight container.
One thing to keep in mind is that when you make your own baking powder this way, it will be single-acting only. What does this mean? Most commercial baking powders are double-acting, because they release some of their gas as soon as the wet ingredients and the dry ones come together, and then the heat of the oven or griddle triggers the release of still more. This is why you can (and should) let your pancake batter rest for 20 minutes to give the lumps a chance to dissolve.
With homemade baking powder, however, there is only one release of gas, when the ingredients are mixed, not during cooking. That means you don't have the luxury of letting your batter sit out.
Rather, once you get it mixed, you need to get it into the oven right away.
Therefore, homemade baking powder is best for quickbreads and muffins and the like, rather than pancakes or waffles. It'll still work for pancakes and waffles, but you have less leeway when it comes to letting the batter sit while you cook.