Designing for augmented reality (AR) applications is difficult and expensive. A rapid system for the early design process of spatial interfaces is required. Previous research has used video for mobile AR design, but this is not extensible to head-mounted AR. AR is an emergent technology with no prior design precedent, requiring designers to allow free speculation or risk the pitfalls of ‘path dependence’. In this paper, a participatory elicitation method we call ‘spatial informance design’ is presented. We found combining ‘informance design’, ‘Wizard of Oz’, improvisation, and ‘paper prototyping’, to be a fast and lightweight solution for ideation of rich designs for spatial interfaces. A study using our method with 11 participants, produced similar and wildly different interface configurations and interactions for an augmented reality email application. Based on our findings we propose design implications and an evaluation of our method using spatial informance for the design of head-mounted AR applications.