I have been looking around the new Blender 2.8 interface now that it is approaching a finished state, and I've noticed that in order to to use the 2D Animation context, you have to actually create a new file specifically in that context. Further, it doesn't seem to be possible to switch from the 2D Animation context to the Modeling, Sculpting, UV Editing, etc. Should I take that to mean that if I wanted to create an animation that blends 3D and 2D--say, a 3D scene populated by 2D characters--that the best way to do that is to create a "General" file and just deal with not having the 2D animation tools quite so easily at-hand?
Another option is to add a grease pencil object in the 3D workspace in object mode.
Go to Add => Grease Pencil => Blank. Then you can switch to draw mode under the ↹ Tab menu. From there you can draw into object surfaces or just aligned with your camera view.
You can select surface and set a low offset so it's not occupying the same space as the face you're drawing on.
This is a great option for drawing details that you can then convert to curves and from there a mesh that you can solidify. It can also be used for stylized texture painting.