Apart from websites, what is currently the principal vehicle used for the emerging field of interactive design?

Handheld Tablet devices

Explanation:

Portable handheld devices and tablet PC's are becoming increasingly popular amongst those who require the assistance and convenience of a conventional computer, in environments where carrying one would not be practical. Tablets, being computers, do what other personal computers do, but lack some input/output (I/O) abilities that others have. Modern tablets largely resemble modern smartphones, the only differences being that tablets are relatively larger than smartphones, with screens 7 inches (18 cm) or larger, measured diagonally, and may not support access to a cellular network.