Show that (a+b)^2 = a^2 + 2ab + b^2 in two ways: using algebra and using a simple geometrical demonstration. Same goes for (a+b)^3.
Ask if we can find a method for writing down (a+b)^n.
Allow for time and experimentation.
There are two ways to go on with this: One is looking at (a+b)^n as (a+b) (a+b)^(n-1), which will probably lead to the recursive definition of Pascal's triangle, and the other is looking at it directly, which will probably lead to the explicit formula for the binomial coefficient.
You can relate this to the general workshop on "n choose k"
A bit more specific about the first two lines:
Start with the following question: You have a square and you would like to turn it into a larger square. How much additional "material" would you need to add to it? A nice geometrical demonstration shows that you need to add two rectangles of equal area and another square. That turns out to be precisely the algebraic identity (x+y)^2 = x^2 + 2xy + y^2.
Now, same question with a cube (here you may use play-doh as a physical example.) Now you get to (x+y)^3.
Now you can explain how these are also algebraic identities and prove them algebraically.
Now ask what about (x+y)^4 [But what does this mean? Is it a hyper-cube? What is this? There is no 4th dimension... {who can help motivate this step?}]