Let us phrase the question in the title in more detail: I wonder if there exists a metric space $X$ which has at least two points, has finite diameter (in the sense that there is an upper bound for the distance between two points, for example it could be compact), is contractible (there is a homotopy of continuous self-maps of $X$ which starts at the identity and ends a constant map) and such that its isometry group acts transitively.
I suspect that there is no such $X$ (maybe with more assumptions, like asking it to be compact or geodesic; if it is CAT(0) then a solution is obvious), but I have not been able to find a proof of this. So I would be grateful if someone knows a solution to or a reference for this problem.
edit: it turns out (see Swiat Gal's answer) that there are such spaces when one drops the compactness assumption--however, I am more interested in the compact case. I also made an assumption to avoid the dumb example of the one-point space.