A fundamental component of mathematical learning is spatial visualization. To improve their spatial visualization skills, students should be given opportunities to work with two- and three-dimensional objects. The authors describe two activities that use tetrahedra nested within cubes to help improve students' visualization of two- and three-dimensional shapes. These activities can strengthen students' spatial visualization and reasoning. The explorative nature of the activities engages students at different levels in active thinking and problem solving. Using well-designed manipulatives such as Zometool and technology tools such as vZome software and The Geometer's Sketchpad helps students develop clear mental images of the two- and three-dimensional geometric objects, which are not easily constructed otherwise, and of their mathematical properties and relationships. Further, encouraging different ways of approaching a problem helps students become stronger and more flexible in working in various problem situations. (Contains 12 figures.)