A major challenge to ubiquitous computing system designers is the provision of walk-up-and-use solutions for supporting activities performed by occasional visitors to a particular place. Ideally, people should perform an activity requiring computing tools as they perform any other activity, by focusing on the activity itself, and using the computing tool as naturally as other tools. There is thus the need to center the design and development of ubiquitous systems in the human activity, in order to transparently support activities that take place in the physical world. This work...