To get ready for this activity, take 5 0r 10 square blocks (or cut squares from foam or paper if you don't have the blocks) and arrange them in a shape on a piece of paper. Trace around the outside of the shape.
Then, I gave B 5 squares (and 10 for the larger shapes) and said, "Here are 5 squares, can you figure out how they fit into this shape?" At first he was putting them inside the shape but not paying attention to the lines. After I showed him how to do a few he caught on was able to do the rest by himself. When we finished he said "I want to do more of these!".
You can also estimate how many squares will fit into the shape first and then check your prediction.
I love how simple this activity is to put together and how you can easily vary the difficulty level.
This activity idea was found at Joyful Learner (link no longer available).