Assuming that Systems of Equations is a review topic for your students, these are optional projects.
The Heating System Assignment (w/Teacher Example) is a great way to review systems of equations while educating your students about costs & decisions associated with owning a home.
And what can I say about the "Soldier's System" (assignment & video below)?...It's a fun excuse to get the students outside and our ROTC students love getting out the gear!