Interactive Area Explorer - Fix the perimeter, and then find the area
http://www.shodor.org/interactivate/activities/AreaExplorer/

Interactive Perimeter Explorer - Fix the area, and then find the perimeter.
http://www.shodor.org/interactivate/activities/PerimeterExplorer/

Interactive Perimeter and Area Explorer - Find both the perimeter and the area of the shape.
http://www.shodor.org/interactivate/activities/ShapeExplorer/