I recently discovered this book by David Adler and decided it would make a great introduction to our geometry study. On the day I used it in class my kids left saying "best class ever!" We really enjoyed it. To prepare, I got a loaf of bread, a package of sliced cheese, a bag of pretzel sticks, toothpicks, plastic knives, and paper plates. As I read the book we did some of the activities.
The first one involved the cheese; using a toothpick we made three dots in the cheese. Then we used the toothpick to connect the dots, which cut out the shape. A few examples:
I quickly learned that we needed to keep the cheese on the plastic wrapper or it would stick to the plate and make it difficult to get our triangle :) After making cheese triangles (introducing scalene triangles) we made triangles from the pretzel sticks - both equilateral and isosceles.
If you are looking for a good book to use when teaching geometry, I recommend this one.