I work in an early education school, and if I had to pick one item that is sure to be a crowd pleaser ages 2 through 5, I would have to say PLAY DOUGH! It never ceases to amaze me how the younger ones are fascinated by its tactile quality, while the older ones can get into some quite elaborate narrative over it.
To make it even more fun, you can make play dough at home. Online, one can search and find tons of recipes which involve elaborate items or some cooking time, but I prefer the simpler version. Here is what most schools I know use as a play dough recipe:
1 cup of flour
1 cup of boiling water
2 tablespoons of cream of tartar
1/2 cup of salt
1 tablespoon of oil
Just mix all ingredients. To make it more fun, add some food colouring! Make sure you are careful if a child is helping you, as the boiling water makes the mixture very hot!