While cooking isn’t strictly a free activity, odds are it’s money you would have spent otherwise, so this one at least doesn’t cost anything extra.
Pick out a new recipe that is age appropriate for you to make with your children. You can get a kids’ cookbook from the library if you want, or buy one if this is something you’ll do regularly enough. And of course there are plenty of kid friendly recipes to make online.
Make it fun or make it part of a regular meal, whatever you want.
Cooking is a great way to teach kids about fractions and other bits of math, in a way that isn’t so intimidating. It can also teach them about responsibility, especially if your child is old enough to start cooking on the stove with you.