And to clarify, I mean as relatively full time work - not something like a professor emeritus that has a class once in a blue moon or a lawyer who is a partner but doesn't really work. I'm thinking of something like an Abraham Lincoln who had been a lawyer and might have gone back to being a lawyer after leaving office (although in that case I imagine he probably wouldn't have even if he had survived)
And John Quincy Adams returned to the Senate
The one who immediately springs to mind is Howard Taft, who was nominated to the Supreme Court and spent the rest of his career on the bench.