One assignment is to use pattern blocks to fill in rectangles. They count how many pattern blocks it took to fill the rectangle.
After they understand that area is the space inside, we learn about perimeter. I explain the perimeter is the FENCE that goes around and the area is the GRASS INSIDE the playground (see my post about singing for the lyrics to the area/perimeter song).
Once they are correctly finding area and perimeter we do a lesson on Chez-It math. It is really fun! Divide the class into groups and give each group a baggie filled with cheese crackers (about 30 crackers per baggie-have a couple students help you fill the baggies prior to the lesson). Call out the areas and perimeters you want them to find (show me a square with an area of 16 units-what is the perimetere, show me a rectangle with an area of 12 units-what is the perimeter, show me a rectangle with a perimeter of 14 units- what is the area?)
Let the students walk around to see how there are several correct ways to get the answer.