Don Steward Algebra Quads
Love this idea by Don Steward, built off of Martin Wilson's idea, of finding quadrilaterals given algebraic descriptions of side length. But it begged to be dynamic, where the quadrilaterals were changing, too.
His original four problem sets are here: http://donsteward.blogspot.com/2016/12/expressions-quadrilaterals.html
The challenges besides the rhombus challenge are to make a parallelogram or kite, or to create three equal sides.