Students are provided rubber poppers (http://www.orientaltrading.com/large-marbleized-poppers-a2-16_454-12-1.fltr?Ntt=popper) that are various sizes and colors. Students design an experiment to test an independent variable of their choice (color of popper, size of popper, surface used to launch, etc.) so they can record the data for their dependent variable (height, distance, volume of 'pop', etc.).
I normally use this as my culminating activity once students have had previous experience with setting up an experiment. This is a practical assessment that allows me to verify that each group of students have mastered the scientific process.
Lab is formatted to be a book-fold insert into their science notebooks.