Everybody likes a thermos to keep things cold or hot, but I have found that people also really like to see what's in a flask and how much there is. (Particularly when I bring my home-grown lemonade to parties, I notice people drink more of it from a clear and non-insulated container than an opaque one.)
One could build a lower efficiency thermos that used transparent insulating materials. But it should also be possible to simply have a transparent tube on the side, joined with the main chamber at the bottom, openable at the top to flush out for cleaning. This tube could be surrounded by transparent but insulating material, and possibly even have a slowing valve to the main chamber so it doesn't mix super fast. The valve would flow mostly one way (into the main chamber) with slow flow into the tube. This way, the room temperature liquid in the less-insulated part would not constantly feed heat into or out of the main chamber.
One could even imagine an entire outer shell on the container with white background, so it looks like you can see a flask of the liquid in question.