With one of my customers, I had a week when I consistently reported "designing the new model for $thing-that-drives-our-product", sometimes additionally with "and I spend some time talking with $product-guy and $coworker about the design". As a side effect of the design process, I produced a bunch of concept documents and drawings, which once the overall goal was clear, I transformed into barebones interactive prototypes testing experimentally some design points. So the next week, my daily reports were "aaand, I also did this prototype to test out $design-concern" (to which the universal reaction was "oooh, shiny! <3"). Two weeks, zero code for the actual product, just couple pages of text and a lot of garbage JavaScript in prototypes.

This is how work sometimes look. A good manager will accept your vague description, trusting that something of value commensurate with time spent will come out from the other end. As long as you actually deliver, a sane manager will not mind.

(That said, if asked for details, I always could explain what my main concerns and areas of exploration were at any point in the design process. The story about what you were doing is proportional to how much work you actually put in. If you can't elaborate beyond "I was thinking about $X", maybe you weren't really thinking about it? Conversely, I find forcing myself to write down and structure my thoughts to be an excellent way to unblock myself when my mind is running circles, or feels like watching Netflix instead.)