Here is a fantastic, soft playdough recipe that, if stored properly, can last for years.
4 cups flour
1 cup salt
4 Tablespoons cream of tartar
8 tablespoons of oil
4 cups of water
Mix all the ingredients in a mixing bowl, add any food coloring you like and cook it in a pan on low heat until it is thick and not sticky. Knead the dough for two minutes. Store it in a plastic bag inside of a plastic container and keep it in the fridge.
This recipe makes a lot of playdough, so you can half the recipe if you want to make less.