Maybe you could have the students build their own vex bot and do a build season, like session where they split off into sub-teams to complete a task, such as a past FRC competition. moving boxes around and then hanging on a bar??
Maybe a Rube gold-berg project which is a series of simple machines linked together so they transfer energy to do a simple task. In my school district the junior high has been doing it for years in it's intro to tech classes all using scrap wood, string, marbles and hot glue.
here are some links (a pic is worth a thousand words)
Pics
Movie This is a movie that has nothing to do with the rube gold berg until Half way through
outline