The balloon rocket racer challenge is a STEM build where students work individually or in groups of 2 to brainstorm ideas, design, and eventually build a balloon rocket racer. The objective is to have the racer go as far and straight as possible. This makes recording data simpler, and usually the designs that go straight are also the ones that go the farthest (although there are always exceptions).
I based this lesson largely from the following lesson idea: http://teachers.egfi-k12.org/activity-ballon-powered-car/. I still use the templates from the website as well as a modified version of the design sheet.