This is a fun STEM or STEAM project that can be used in a variety of classes. The goal is to create a Ping-Pong ball launcher. Each material is given a price (there are 2 price sheets - One has "pretty numbers" like $2.00 while the other uses decimals like $1.95) Depending on the level of your students I would encourage you to tell your students to add a sales tax to their materials and included are coupons that students can earn that take a percentage off their totals.
Along with engineering and problem solving skills, this project can include calculating percentage, budgeting, and check writing.
I love to use this on "fun days" such as the first week or last week of school or days during standardized testing. This is a fun way for the students to do math in a very hands-on way!