We describe a tangible user interface-based learning environment for children called Towards Utopia. The environment was designed to enable children, aged seven to ten, to actively construct knowledge around concepts related to land use planning and sustainable development in their community. We use Towards Utopia as a research prototype to investigate how and why tangible users interfaces can be designed to support, augment, or constrain learning opportunities. We follow a design-oriented research approach that includes a theoretically grounded analysis of design features of Towards Utopia to understand how and why design choices influence the kinds of learning opportunities created. We also describe the results of our empirical evaluation of learning outcomes in order to validate the effectiveness of our design. We conclude with general guidelines for the design of tangibles for learning.