Collect a selection of objects from around your home, these should be lots of similar objects, e.g. cuddly toys, building bricks, spoons, colouring pencils etc.
Look carefully at each one and decide how to sort them into sets. Try to sort the same objects into different kinds of sets and discuss how many objects are in each set every time. Which set has more or fewer objects?
You could sort by colour, size, shape, material or in another way.