I've been reading some of John Baez's work on 2-categories (eg here) and have been trying to visualize some of the constructions he gives.
I'm interested in coming up with 'concrete' examples of 2-categories. As an example of what I don't mean, I know that the category Cat forms a 2-category, where the objects are small categories, the morphisms are functors and the 2-morphisms are natural transformations. But this is too abstract for me - given that categorical constructs are what I'm having trouble understanding, it doesn't help me much to give an example from category theory!
One thought I had is that you might be able to view a group as a 2-category. Taking the perspective that a group is a category with one object where the morphisms are the symmetries of the object, you should then be able to construct a 2-category by saying that the 2-morphisms are the inner automorphisms of the group. An interesting question is then what the compositional structure of the 2-morphisms is.
To be really concrete, consider the group $D_3$. Here the object is an equilateral triangle, and there are six morphisms $e$, $r$, $r^2$, $m$, $mr$ and $mr^2$ where $e$ is the identity, $r$ is rotation by $2\pi/3$ and $m$ is reflection in one of the axes of symmetry, and the others are the obvious compositions of these.
Then the 2-morphisms are the functions $\phi_g$ given by $\phi_g(h)=ghg^{-1}$. For this example, the 2-morphisms have the structure of the underlying group $D_3$, but clearly this isn't always the case (e.g. for any abelian group the 2-morphisms have the structure of the trivial group). I haven't worked through many of the details, but it seems like there might be the grain of an interesting line of thought here.
So my questions are:
Is viewing groups as 2-categories an interesting thing to do, i.e. does it give you any new perspectives that make previously esoteric facts about groups 'obvious', or at least special cases of results in 2-categories?
What other 'concrete' examples of 2-categories are there?