As long as you don't mind a rodent eater i would advise a western hognose. They are small enough, usually real calm, and no threat to a anything bigger than a mouse. They are also easy to keep almost always and widely available CB. Oh, and they are diurnal which is nice if you would like to see your snake do anything besides try to hide under things.
I would also mention out that Candioa have something of a reputation as hard to care for.
Egg eaters also look to be alot of fun, they are what I am looking around in this forum to read up on.

good luck