I did this lesson a couple of years ago with my 7th and 8th grade students to go over properties of a circle. They had a lot of fun with the activity and it was much more interactive than just doing practice problems off a worksheet.
Students blow bubbles onto large construction paper. When the bubble lands and pops it leaves a circular ring, which the student quickly traces before it dries.
We learned that the lesson works better if students use multiple types of wands, otherwise the bubbles turn out to be pretty similar in size.
Once student traces a set number of circles (we did 10) they then find the radius, diameter, circumference, and area for their bubble imprints. To give a greater variety of numbers, students were asked to choose different units of measure throughout their poster: centimeters, millimeters, and inches.
*Would also be a good lesson to use for finding original volume of the bubble.