Plan B: The Campaign for the Cabinet

The first presidential primary votes won't be cast until January, and the Republicans won't nominate their candidate until July. Although almost anything can happen between now and then, most of the remaining 15 GOP presidential hopefuls probably already realize they don't have a serious chance at winning.
So, why are they still running? Could it be that their end-game is not, in fact, the Oval Office, but rather some other plumb post? Do even the serious contenders have a Plan B?