For me, both “tangible” and “interfaces” suggest specific connotations. “Tangible” typically imparts a sense of an object, that can be handled physically, while “interfaces” suggest a kind of bridge between different spaces or dimensions, the digital and physical, for instance, and usually, a more limited set of options and possibilities.
I am particularly interested in how we apprehend physical and social spaces, and the elements that go into constructing these physical and social spaces. We typically conceive of space as an environment that offer a wide enough (often infinite or near infinite) array of possibilities, such that agents have significant freedom and flexibility. In physical space, the ability to move across three planes of movement (i.e. x, y, z) offers this infinite or near infinite range of possibilities. In this sense, space can be constructed by any material (or some combination) that offers such a wide range of possibilities. In many purely virtual environments, it is text, or words, that simulate a sense of space by offering such an array of possibilities. Many other building blocks, beyond text or three-dimensional space, may offer this freedom of movement or creative expression. 
Objects, interfaces or architectures that impart or afford a sense of space are fascinating because they tap into a ubiquitous, but perhaps less noticed, aspect of how we apprehend and construct our identities, agency and how we relate to one another. Temporarily setting aside architecture, objects inserted into shared or public spaces can dramatically alter how that space is perceived, opening up new affordances and new dimensions along which one may act and interact, broadening the social space beyond its physical dimensions. In this context, I found activity theory compelling, particularly in that it “maintains no properties of the subject and the object exist before and beyond activities”, and second, where “activity is considered the key source of development of both the object and the subject.” The latter characteristic is particularly interesting when we consider the effect the design of shared social spaces can have on our identities, assumptions and learned behaviors as citizens, residents and participants in any system or society.
One of my favorite UIs (if it can be called that) is the urban sidewalk. It functions both as a means or mode of transport, on the way to some place else, but also a public space and potential destination in its own right (such as when people go for a walk). Its relatively simple characteristics – its width, cleanliness, proximity to other locales, safety, slope and evenness lends itself to a variety of affordances, a curb allows you to easily sit, hard surfaces for exercise or play, potentially low-cost and informal socialization, a slope allows a thrill ride for cyclists/skateboarders etc. In sidewalks and other public spaces, I am curious whether inserted objects can radically change how people perceive and interact within that specific space, as well as influence how they perceive themselves and their roles within the other overlapping groups in which we are embedded.
1. Definitions of “space” borrow heavily from my Computer-Mediated Communication Final Paper on Collective Neutral Spaces (2016).