I can't remember when I first saw this... perhaps at a California Math Council conference session. As an alternative to wrapping string around a circle or a shape, students can draw a line on a big sheet of paper and roll objects along it, carefully marking the start and end points. Then they can mark out the diameter along the line. With the measurements in hand, you can calculate and compare the ratios, and/or mark out three diameters along the circumference for a great visual of "pi is 3 and a little bit more."

I did this as a Pi Day activity, and my students clearly benefited from revisiting circumference with strings again later in the year.