WristLens is a system for surface interaction from wrist-worn wearable devices such as smartwatches and fitness trackers. It enables eyes-free, single-handed gestures on surfaces, using an optical motion sensor embedded in a wrist-strap. This allows the user to leverage any proximate surface, including their own body, for input and interaction. An experimental study was conducted to measure the performance of gesture interaction on three different body parts. Our results show that directional gestures are accurately recognized but less so for shape gestures. Finally, we explore the interaction design space enabled by WristLens, and demonstrate novel use cases and applications, such as on-body interaction, bimanual interaction, cursor control and 3D measurement.