Well, let's begin with Republican candidates other than Trump, then move to other conservative parties and independents.
First off, Republicans: Bill Weld, Bob Corker (unofficial), John Kasich (unofficial).
Now onto others. The Libertarian Party: Adam Kokesh, John McAfee, Arvin Vohra, and possibly Sam Seder. And also, possibly Justin Amash.
American Solidarity Party (I personally don't think they shoudl exist, but that's out of the question): Brian T. Carroll, Joe Schrider, Joshua Perkins.
Prohibition Party: C.L. Gammon.
And, for the sake of completion, I will include a Libertarian-Party candidate who has a reputation for being not at all serious, and an independent whose policies I am unable to find: Vermin Supreme, and Ronnie Kroell, respectively.