Smartwatches are characterized by their small size designed for wearability, discretion, and mobileinteractions. Most of the interactivity, however, is limited to the size of the display, introducing issues such as screen occlusion and limited information density. We introduce Wristband.io, a smartwatch with an extended interaction space along the wristband, enabling (i) back-of-band interaction using a touchpad, (ii) a low resolution ambient watchband display for offscreen notification, and (iii) tangible buttons for quick, eyes-free input. Insights gained through a study show that back-of-band input increases accuracy and task completion rates for smaller on-screen targets. We probe the design space of Wristband.io with three applications.