Polystyrene foam, also known as Thermocol or styrofoam, is a lightweight packing material used to protect fragile instruments from damage. Instead of throwing out your old Thermocol, why not recycle it by making a model of an animal cell? Making a model is a great way to understand any complex system, and while the animal cell is one of the most basic units of life, it is more complex in its workings than any man-made machine.
Visualise the parts of a cell and an easy way to construct them. An animal cell has a cell membrane, a nucleus, mitochondria, rough and smooth endoplasmic reticulum, ribosomes, and countless other parts. It's up to you to decide how in depth you want to model. Keep in mind that there are many different types of animal cells and each of them is very different---liver cells have more extensive endoplasmic reticulum, neuron cells have a very unique shape, etc animal cells have near endless variety and range in size from skin cells smaller than a period to gigantic ostrich eggs.
Find a nice box shaped thermocol piece for the cell membrane. You can make holes in it to show that it is porous, or stick pins or glue lego pieces onto it to show the receptors and membrane proteins if you want--the cell membrane has many attributes functions and its up to you to choose which ones to emphasise.
Start adding in some organelles. Strips of cardboard arranged in a folded up way are great for structures like mitochondria and endoplasmic reticulum, which require a high surface area to function. Lysosomes, little capsules of digestive aids, can be represented by those little plastic Easter eggs that can be opened. You can stick little pieces of clay to the rough ER to show ribosomes. You can even stick little strings into the clay to show that they are processing RNA into proteins.
Compartmentalise some of the organelles with smaller thermocol boxes--for example, represent the nucleus as a little box with strings for chromatin. You can also represent irregularly shaped organelles, centrioles for example, by tearing or cutting chunks of thermacol into the approximate shape and painting/colouring them a new colour.
Add finishing touches. Make good use of colour coding to delineate different organs and parts. Consider printing out small labels to stick on to the organelles. A good way to stop loose organelles from rolling around is to stick them on with tape or glue. You might want to stick the smaller organelles on with toothpicks--this makes them easier to see and helps stop you from accidentally crushing them while attaching them.
Once you have a model, put it to good use. Show how material travels from one part of a cell to another, how food is digested, how DNA is transcribed and translated, how proteins are formed, etc.
Don't give sharp objects to children who are not mature enough to handle them and provide the appropriate amount of supervision.